EFHW Measurements coupler comparisons

I wanted to compare my T-50-2 coupler with my T-200-2 coupler. The measurements did not line up with what I had measured previously.

Antenna:  66ft 8inch wire in my garden. Vertical approx 20ft. Horizontal approx 38ft. Other end hanging down 8ft.

Resonant frequency: 7020Khz

Coupler 1:  T-50-2. 28:3 turns. Impedance ratio 10:1 (50*100=5000ohms). Resonated with a variable capacitor.

• Counterpoise 1: 6ft 8inches.  SWR 1:1. 2:1 BW to 7.1Khz. (0.06wavelength)
• Counterpoise 2: 23ft 1inch.   SWR 1:1  2:1 BW to 7.1Khz  (0.17wavelength)

Coupler 2: T-200-2  18:2 turns. Impedance ratio 9:1 (50*81-4050ohms). Resonated with a length of coax.

• Counterpoise 1: 6ft 8inches SWR 1:1. 2:1 BW at 7.080Khz (0.06 wavelength)
• Counterpoise 2: 23ft 1inch   SWR 1.8:1. (could not achieve a good match)

In both cases the most stable system was achieved with the short counterpoise.

Both systems exhibited roughly the same bandwidth. (this is different to what I have seen in the past where the longer counterpoise resulted in a wider bandwidth??) 

NOTE: In the case of the T-50-2 the secondary was tied to the primary at the cold end.

Calculated inductance for Coupler 1. ~ 1.8 uH

Calculated inductance for Coupler 2 ~ 4.0uH

Capacitance to resonate Coupler 1. ~ 285pf

Capacitance to resonate Coupler 1. ~ 128pf

Coupler 1  Xc = 79ohms = Xl  (Q= 5000/79 = 63. not sure if this the correct calc?)

Coupler 2 Xc = 177ohms =Xl (Q= 4050/177 = 22. )

However the problem with the above calc is that the input impedance to the antenna/counterpoise must change with different length counterpoises?? See fig3 of the AA5TB efhw writeup.

More to think about here? No wonder this antenna is not well understood!

W1FB DC Receiver experiments 5

This morning I was listening on my receiver at 6:45am - 8am. This was interesting. When I started listening I could hear DX stations from the USA coming through although not strong. By 7am the band was shorter and I could hear Capetown stations working into Joburg. By 7:30 I was hearing loud BCI. I don't have a shortwave general coverage receiver but I got the impression that these were not mediumwave AM stations but rather shortwave stations perhaps on the 41m and 49m bands. My BCI filter was not filtering out these stations obviously due to the fact that this is designed for filtering BCI below the shortwave band. The stations were not tunable except that by adjusting the input circuit I could definitely significantly affect the loudness of this BCI when compared to ham stations. The BCI was indeed coupling through the antenna (I think). When I disconnected the antenna the BCI went away.

I could also hear band conditions changing over this period with QSB on local stations and also the CapeTown stations. Based on the content of the BCI I was hearing southern african broadcasts in addition to local south african shortwave stations.

By 8:10am I was hearing BCI only from a local South African station and the african stations had gone.

So in conclusion I have 2 different kinds of BCI to contend with. The AM medium wave is sorted by using the BCI SW bandpass filter. The shortwave BCI probably needs better front-end filtering to eliminate?

However at this time of year all BCI disappears by 9am.

Superglue on toroids

Last night I decided to put superglue on the oscillator toroid to see if I could stabilize it mechanically. What I found was interesting. It is easy to apply it to the windings and to the area between the core and the underneath copper....no problem. It is very runny and tends to run a bit. However it dries very slowly when not under pressure. By this morning it was dry and appears to have worked. The toroid is now stable to mechanical vibration. The glue seems to be dry. The inductance of the toroid appears to be unaffected. Not too sure if the Q has been affected however.

W1FB DC Receiver experiments 4

This evening I connected a BCI filter that I have on hand, in the antenna path to my DC receiver. This is a unit I purchased a number of years ago from Industrial Communication Engineers. Model 402X.

The spec as follows:

BCB filter.  1.8Mhz - 30Mhz

50 Ohms, Non-Polar

Non DC Passive

Power capability is 300Watts.

The filter completely eliminated the BCI. Gone 100%.

This tells me that the BCI is entering through the front-end.

I can tell it is AM broadcast since the interference it is non tunable. A very loud Portuguese speaking station this evening.

I may consider building a BCB filter for my field project. However this will take up real estate and will need some kind of shielding to work properly.

BCB Filter from Industrial Communications engineers.

W1FB DC Receiver experiments 3

Today I tried to calm the receiver a bit since the audio gain control has to be turned almost to zero with the original W1FB design.

Using my DMM I noted that the input voltage to the Audio Preamp was about the same as the input voltage to the LM386 for acceptable listening levels! Meaning that in practice the pre-amp is not achieving anything and is increasing the noise level.

The audio pre-amp is designed to swamp the noise figure from any added audio filter stage. These generally being noisy in nature.

I simply connected the NE602 directly into the LM386 and bypassed the preamp. This has resulted in much less gain (of course) and a better behaved receiver. Next I removed the 1K feedback resistor in the LM386 to obtain a  little more gain.

I also noticed a big difference between my senheiser earplugs and a pair of cheap earphones. Amazing the difference. The cheap pair are better since they do not hear the high frequency hiss in the audio amp.

I need to next increase the .1uF cap on the audio output and see if I can reduce the bandwidth a bit.

W1FB DC Receiver experiments 2

On 5/25/2013 in an effort to improve stability and reduce the tuning range, but at the same time restricted by components on hand, I reworked the oscillator circuit as per the calcs shown below. I was able to reduce the tuning rage down to about 80Khz from the previous 500Khz. This makes it much easier to tweek the trimmer cap with a tuning stick. The inductor is now also much more stable due to the increased inductance to 1.85uH from the previous 1.3uH. I also used thicker #22 wire and pulled it tight on the toroid. These two changes have actually made the receiver practical to use in the field.

My aim next is to acquire the right components and rework the design again to the original spec. It seems difficult to obtain NPO caps here in South Africa. Perhaps I will try some SMD components? The inductor should be at least 2.6uH to achieve the recommended L/C ratios.

W1FB Direct Conversion Receiver experiments 1

As a part of my project to build a field radio NVIS system, the scope of which I have described in previous blogs, I have now built a first version of the W1FB Direct Conversion Receiver. This venerable design is fully described in W1FB's QRP Design Notebook so I won't repeat it here. I built my version using Ugly style construction and specifically Manhattan style. This being a pleasure and possible with only a hacksaw to cut the PCB and a pair of side cutters to cut out the 'islands'. See a pic below.

My challenge was that I did not have the same components for the Colpitts/Clapp design Oscillator. So I set about seeing what I could do with the components on hand. Certainly I did not have a T-50-6 material toroid. I used a T-50-2 instead. This core is not designed for temperature stability use but I had no choice. The trimmers I have are also probably not suitable and are SMD type trimmers. In addition the capacitors are all ClassII X7RO types. Oh well I will learn something for sure.

The calculations I did are as per the included paper below. I found a calculator online that helped greatly and eased the tedious maths a bit. I also measured the actual inductor and capacitor values using my L/C bridge. Once I had the oscillator built I then used my MFJ259 to verify that indeed the circuit was resonating in the 40meter band. My half turn trimmer covered from 6.6MHz to 7.2Mhz. This was going to be a challenge to tune!

I started by building the LM386 based audio amp. Verified that it was working by simply touching the input with my finger and hearing hum in the earphones. I then added the Audio preamp and conducted the same test. I certainly could hear added gain and louder hum in the earphones. The components were all as per W1FB's design except the speaker coupling capacitor which is a 100uF unit. I supposed that the low frequency response is less as a result of this.

I also have not yet built the audio filter, figuring that the mission was to just get a platform design working.

My design has a smaller inductor of 1.38uH and is inherently less stable.

When I first turned on the rig I heard nothing. I could peak the noise on the tuned circuit so I figured that it was working. I then connected my 2meter J pole to the input and could hear a few very low level signals. I then transferred to the freezing patio outside where my 40m efhw is located. Man was it cold and now 2am in the morning. But I had to see if I could hear anything.

I connected my efhw and put in the earplugs. On turning on the rig I was greeted with a very loud blast of sound! I could not believe it! On turning the volume down to a minimum and tuning the oscillator with a tuning stick I was able to copy loud CW and SSB signals coming in from Europe and the US. Excellent! Over the next few days I tested the rig at all times of the day. During the mornings there is loud local broadcast interference. During the afternoons the rig works fine except that the bandwidth is very wide (not surprisingly). The dynamic range of the NE602 seems very good. Tuned to the CW section of the band I can hear local SSB up frequency. So there is plenty of room for improvement. Adding and audio filter will greatly improve daytime operation. I am less sure about how to cure the BCI however without adding excessive filtering on the front-end. Microphonics seem under control and also there is no hum. all good. There are 4 areas for improvement in the following order of priority.

1. Improve the VFO. First arrange the capacitors to allow narrow band coverage with the trimmer. This is going to be tough since only a few picofarads are needed to tune the 40Khz CW band. Second I can do some things to the physical construction to improve stability. Mainly securing the inductor and windings better. Having said this I am very happy with the short stability as is. Once the rig is tuned on frequency there is no drift discernable during the period of  a typical QSO.
2. Improve the selectivity by adding an active audio filter. This filter should be optimized for CW reception and should eliminate the higher band ssb interference from loud local stations.
3. Improve the front-end selectivity by perhaps adding a double tuned front-end parallel circuit. This should help eliminate the SWL broadcast interference when the band goes long in the evening.
4. Put in a BCI filter and shield the receiver to eliminate the very strong BCI that swamps the radio in the mornings. 

W1FB DC Receiver

ZS6RSH version 5/24/2013

ZS4SF field EFHW and coupler adjustments

Om Monk sent me this email of his adjustments for his field coupler for his EFHW. Included here per kind permission of ZS4SF.

Howdy Dick,
here is how I tuned the antenna.
Used  T68-2  toroid.   20  Turns  for  the  secondary, 3 Turns for the
primary. Connected a variable cap across the secondary.
Adjusted  swr  to  1.2  to 1, removed the cap and measured it,, 130Pf.
Installed  fixed  cap  of  130Pf and tested. Swr high.Tried small caps
across  but  could  not  find a match. Connected 6" of coax across and
tuned to 1.2 to 1.

Started spacing the coils on the core till 1 to 1.
Pushed  everything  into  the film can. Swr now 1.5 to 1. Removed from
can  and  got  Swr back down. Rub glue over core and wait to dry. Back
into  can  and  again  swr  high  (  1.4  to 1 ) Have a look see where
resonant point is ( 6. 9 Mhz ) Antenna too long, cut abt 5" off and swr
at 7020 now 1 to 1.

Bandwidth is as follows: 7000 to 7065 1.1 to 1
7065 to 7130 rises up to 1.5 to 1
7130 to 7200 rises up to 2 to 1
This is good enough for me so left as is.

Points to note:
Spacing the windings is sensitive,
Stray capacitance a factor to consider
It is better to leave a short piece of coax across the secondary so as
to fine tune at a later stage.
QRP Antenna now ready for field work  Hi!

73
Monk
PS   Antenna length to start with  66 feet 8"
I found that the antenna could be tuned with 27 turns on the secondary
but  knowing  that more inductance and less capacitance makes for less
bandwith.  i am sure that 1 to 2 turns can still come off but as i was
satisfied with the bandwidth I left as is.
.5mm formex wire for secondary and 1mm pvc insulated wire for primary.
CU on the air.

Film can cannister open and showing the fixed cap and coax  variable capacitor

Sealed coupler ready for field deployment. How neat is that!

Follow up information received from Monk:

Hello Dick,
Top  of  the can too weak for my liking, changed to plastic water pipe
with  end  blanks.  Some  what  larger but more robust. Attach rope to
middle section and hoist up, ant horizontal.
Bandwidth  now changed,7000 Mhz to 7080 1.1 to 1 : 7080 to 7110 1.5 to
1. I am happy, don't need higher,no ssb on planned antenna.

Manyane Campsite Pilanesberg National Park

We had the usual great time at Manyane this past weekend where we hooked up with our friends Tony and Pat. They have a great camping trailer and we enjoyed  a number of excellent meals and a good social time there. The weather is still quite decent and mild so no problem staying warm.

From a field radio perspective, this was also a good fun learning experience. The sun spot number was reportedly over 200 which I believe is for the first time this cycle (I am not certain about that). The conditions were thus a bit different on 40meters.

I took along my prototype T-200-2 coupler (see pics below) and was able to preform extensive tests with ZS4SF, my friend Monk in Welkom. Path distance would be an  estimated 400-500Kms?  On Saturday morning I deployed both the 31 ft quarterwave counterpoise along the ground and a .05 wavelength 6ft 6 inch counterpoise. My EFHW was higher than usual. The horizontal part was about 45ft long and at a height of about 20ft. The last 20ft sloped down to my operating position. The band was a bit unstable and thus QSB did play a part. The 31ft was receiving very slightly better reports than the 6ft 6inch counterpoise but the results really were very similar. The only difference was the same as I had seen at home and in my previous park deployment and that was a significant difference in bandwidth.

I was very pleased to note that I had no problems loading up the efhw and achieving 1:1 SWR with both counterpoises. No problem at all.

On the Saturday AWA net I received lower signal reports than usual. Mostly 579 and using 5 watts. I don't know why this was? It could be that the antenna was a bit too high for NVIS work? Or the high sunspot number was affecting propagation? A station from CapeTown om Adrian did call into the net and I copied 549. However he was unable to copy me. He was able to copy the Joburg stations but down in the noise.

On Saturday afternoon I erected a 10m dipole at 25ft and was able to copy quite a few DX stations. The band was mainly open to the west and this was the direction that my dipole was deployed. I managed to work one station in India on SSB. I was running about 8 watts. He gave me a satisfying 57 report.

This was also the first time I got to try out my new efhw with the launch cord connected directly. I smoothed over the joint with plastic steel. This makes it easy for the system to slide through the branches. I need to get one more length of the smooth light cord since this slides over the branches so much easier than the thicker cord. The only negative is that if a knot is pulled tight it is much more difficult to undo the knot with the thin cord. I am not sure how to improve this factor.

I also tried out my new modular dipole system. I took only a single center connector/hub. Then I was able to change the dipole lengths accordingly by screwing the different quarterwave lengths into the chocolate strip using my leatherman knife. I found this time consuming and a pain. Give me a doublet any day!

Observations and future actions.

1. The efhw fixed component doubler definitely works in the field without having to tweek it. Next adjust the smaller T-50-2 coupler using fixed components.
2. The quarterwave counterpoise results in greater bandwidth than the 0.05 wavelength counterpoise, however the performance on the air is the same.
3. An efhw deployed higher does not appear to really improve local (NVIS) communications when compared to a 10-15ft high configuration.
4. The new fishing weight launcher is pure pleasure to use and I cannot think of any way to further optimize it. This launcher can be easily tied to any cord using a reef knot at about 3 ft length.
5. The modular dipole system using a single hub to reduce weight is time consuming to erect. Consider using a single wire that is marked for resonance on each band and that is simply rolled onto two shuttles at each end. This system I remember was used in the old days by the Rhodesian Army Signal corp with good success although it is heavy.
6. A coil of wire at the end of a counterpoise does not materially affect the length of the counterpoise. Uncoiling a wire to different lengths and leaving the remainder as a coil at the end makes sense.

I still don't have a good method to sucure the inverted L at the 'knee' point. This trip I tied a knot in the efhw. This came under some strain and I noticed that the pvc insulation had been slightly damaged. How can I solve this problem in future?

My Field gear for this trip. 

Click on pic to enlarge.

Top Left:

1. ~ Quarterwave counterpoise for 40m. 31ft in length.
2. ~ 0.06 wavelength counterpoise for 40m. Approx 6ft 6inches.
3. Four lengths of 20meter slippery cord including the fishing weight antenna launcher.
4. Leatherman.
5. Microphone for the K2.
6. Cable for connecting to the CW Paddles.
7. CW paddles folded away. Palm Mini paddles from Morse Express.
8. Altoids tin variable QRP coupler for EFHW antennas
9. RG58 coax. About 20meters.

Center Left:

1. Short coax connector  for coupler connection.
2. Earplugs.
3. Diploe center hub with 40m dipole connected.
4. Quarterwave wires for 20m, 15m and 10m. Can be attached to the diplole center hub using choc strips.
5. 12V battery cable for backup connection to car battery. (never been needed yet).
6. A bag of different BNC and PL259 type connectors

Bottom Left:

1. Pen.
2. Small notebook.
3. Box with prototype T-200-2 coupler
4. Elecraft K2 full featured Kit Transceiver.
5. K2 custom built carrying case.
6. Small backpack for all the gear in this pic (except the K2)

  

Only in South Africa will you see cooking camp ware like this :)

Tony doing his thing. This is a serious breakfast buddy!

Prototype high power efhw coupler using a T-200-2 Toroid and 115 pf coax capacitor.

Berry, Pat and Tony alongside the camping trailer. Superb!

Pilanesberg National park. Click to enlarge.

EFHW Experiments continued

Further to my EFHW experiments. 

I have now performed some measurements on my system as follows:

Fres = 7020Khz

Toroid = T-200-2

Capacitor = RG58 cable ~ 115pf

1) Primary link turns = 3.5

1.5:1 Counterpoise length = 31ft (I could not get the SWR down below 1.5:1)

Fmax at 2:1 = 7.24MHz

Bandwidth = 220*2=440Khz

2) Primary link turns = 2.5

(Turns ratio = 18/2.5 = 7.2:1. Impedance = 50*7.2*7.2=2592ohms)

1:1 SWR Counterpoise length = 31ft 

Fmax at 2:1 = 7.15Mhz

Bandwidth = 130*2 = 260Khz

3) Primary link turns = 1.5   

(Turns ratio = 18/1.5 = 12:1. Impedance = 50*144 = 7200ohms.)

1:1 SWR Counterpoise length = 5ft

Fmax at 2:1 = 7.090Khz

Bandwidth = 70*2 = 140Khz

If you have a short counterpoise the bandwidth drops, and the input impedance rises.

Why?

EFHW Experiments continued

I now experimented with different combinations of Coupler link turns and counterpoise lengths but keeping the antenna wire constant at 66ft 6 inches. I am now starting to get a feel for the variables.

I shortened the length of the counterpoise to about 6 feet. After slightly adjusting the resonant frequency for 7020Khz by adjusting the turns spacing on the toroid I was then able to improve the SWR down to 1:1 by reducing the turns on the primary link winding down to two turns. However the bandwidth was reduced significantly (I did not measure it) and the circuit was very difficult to keep stable as far as resonant frequency was concerned and as far as SWR was concerned. Small changes to the inductance resulted in big changes to the resonant frequency and SWR. I then went the other way and increased the turns to 4. With this setting I had to increase the length of the counterpoise to over 25 ft in order to achieve an SWR of 1:1. However the circuit became much more stable and the bandwidth expanded to easily cover the whole 40meter band.

So what is happening? Which setting is optimum? Does it really matter?  Looking at the AA5TB Figure 3 graph I note that as the counterpoise is lengthened that the end fed impedance would rise towards 5000 ohms. This would mean an increase in turns ratio would be needed to get to 50 ohms. So for 18 turns on the secondary I would need 1.8 turns. If I shorten the counterpoise then the end fed impedance would drop to say 1800 ohms. So for 18 turns on the secondary I would need 6:1 ratio which would mean 3 turns.

This result does not appear to make sense. Will think on this further.

EFHW Coupler experiments contd/

I erected a wire in my garden that is exactly 66ft and 8inches long. The shape of this wire is an inverted U. This is also known as a HALF SQUARE. Theoretically this resonates at 7020Khz although in practice it is probably affected by the surrounding trees etc. I have not tested the actual resonant frequency of the wire. I guess I can do by coupling it at the end in some way into my antenna analyzer and using it as a grid dip oscillator. I then coupled it to my T-200-2/RG58U coupler. The Toroid has 18Turns of large stiff wire on the Secondary and the link has about 2.8 turns of the same stiff wire. The Link is not connected to the secondary since I wanted to experiment with a counterpoise. I connected the antenna analyzer to the link coil and was able to tweek the windings on the toroid to obtain resonance on the target freq 7020Khz. I did not find it necessary to change the coax length. Resonance was observed by a slight dip in the SWR meter.

I then connected the counterpoise to the cold side of the toroid winding and by unwinding a length from the coil I was able to observe a change in the SWR as it dropped lower. I noted that the length of the counterpoise did not materially affect the resonant point. Only the SWR. The SWR continued to decrease as I rolled out the wire on the lawn. I placed the coupler and the analyzer on the lawn and on the garden table. In both cases I was unable to detect any difference in readings.

When I rolled out approximately 21 foot of counterpoise I noted that the SWR dropped to 1:1. Upon rolling our more wire I noted the SWR begin again to increase. The optimum point was quite sharp to note. So what is going on?

The counterpoise length has a major impact on the SWR but only minor impact on the system resonant point.    Looking at Fig 3 of the AA5TB graph it doesn't make total sense although the reactive component of the model does stay approx constant between .05 and .45 wavelength at 1000ohms reactive. Perhaps this was the part of the curve I was operating in. The optimum counterpoise length seems to be about .2 wavelength with this set up.

So what? This looks like a practical setup. Can it be repeated in the field with different wire configurations? This needs to be tried next on my next field trip. If the coupler can be adjusted to resonance by tweeking the turns and if the SWR can be adjusted by rolling out an optimum length of wire then we have a practical system that can be deployed and that can be used for high power. This system does not rely on an expensive transmitting capacitor.

Next in my garden I will try to see if I can get the system to stabalize by using a counterpoise of 6ft 8inches. According to the AA5TB figure 3 model that length is the length where zero reactance ocurrs. This is the 0.05 wavelength point. What procedure would I follows?

1. Attach a 6ft 6inch counterpoise
2. Bring the system to resonance on 7020Khz by tweeking the toroid coils on the toroid.
3. Tweek the link coupling for lowest SWR. (This may be easier said than done but let's try).
4. Check to see if touching the counterpoise or the coax feeder that the system is stable.

Saamrus Magaliesberg and EFHW revelations

We went to one of our favorite places for the weekend of May 11th and 12th, Saamrus in the Magaliesberg. This is an area where we can go on some great walks and let our dog run free. What a great time we had although a little cool.

Om Monk ZS4SF had mentioned to me that he thought that I had a problem with my EFHW set-up since I had measured the coupled wire as being approx 62 feet long. Meaning it was resonating at around 7.5Mhz. When I arrived at Saamrus I measured and adjusted my new thin wire EFHW to exactly 66ft 8inches. Since the weather was cold I was forced to sit inside in the small kitchen. I set up my K2 on the kitchen table and had about 5 feet of the EFHW inside. The far end was launched up at a height of about 25 feet into a tree. My new launcher worked beautifully. The slippery thin cord had no problem sliding over the tree branch and the weights dropped the far end to the ground without incident.

On connecting my T-200-2 coupler to the EFHW I could not get a decent match at 7020Khz. The SWR was basically off the scale. The resonant point for the system was at 6.8MHz where the SWR was down at 1.2:1. I then trimmed a few inches carefully off the RG58 coax that I was using as a capacitor and was able to bring the system to resonance at 7020Khz. I then noticed that the system was still unstable. When I touched my hand to the rig the SWR varied wildly. However the resonant point remained much the same. I then connected about a 20ft piece of wire to the ground connection on my K2 and ran it along the concrete floor. This made a huge difference and the system became very stable. I then increased the Link coupler by another turn to about 2.8 turns. This brought the SWR right down to 1:1 and a stable system.

I noticed also that the Bandwidth of the system was much better and that I could tune right up to 7.2MHz and still be in under 2:1 SWR. Why was this?

The next morning, Saturday, I erected my portable 7m mast and deployed an inverted Vee 40meter antenna. Fed at the center with a coax cable. The SWR was a bit high at 2:1 on 7020Khz and I did not have a Balun.

On a QSO and exhaustive tests with ZS4SF I received slightly better reports from the Inverted Vee. Propagation conditions were very bad with high QRN and much QSB. The conclusion was that the report differences were marginal at best. I did notice significantly more noise on the EFHW as would be expected. The Inverted Vee was nice and quiet.

The coupler new turns ratio was 6:1. This means that the impedance seen at the end of the wire was more like 50*36=1800ohms. I could verify this by connecting 1800ohms across the coupler and measuring the SWR. According to the books with a counterpoise of 20feet the end resistance should be more like 5000ohms. Why the difference?

When I called in to the AWA net at 2pm local I was disappointed to note that I was receiving 579 reports. After the net I discovered that I did not have the counterpoise connected. Ok so you can still communicate but the difference was at least 2 S points down (12db).

On Sunday morning I had a great ragchew with Barrie ZS6AJY and ZS5SF who both gave me 599 plus reports. FB.

Next steps: I will adjust my small T-50-2 field coupler to resonate at 7020Khz. The link coupling needs adjustment to reduce the turns ratio. This is a huge lesson learnt for me. I will experiment with different length counterpoise wires in order to establish a stable deployment scenario. Next weekend we will go to Manyane again. This will be an opportunity to test the new field system.

View of Inverted Vee 40m dipole and the Saamrus cottage 'Piet my Vrou'

ZS6RSH. Rugged explorer at the summit

T-200-2 Coupler used in EFHW Experiments

Based on comments and experiments from Monk ZS4SF, I was interested to see if I could build a larger coupler for my home deployed EFHW using a large toroid T-200-2 which I happen to have on hand on my work bench. This would allow me to use higher power and in theory would be more efficient than my field coupler which uses a small T-50-2 toroid. I learnt a lot during the process.

I realized by looking at the XL/XC reactances at my target resonant frequency of 7020Khz that the challenge was going to be the turns ratio and small capacitance. I needed a 10:1 ratio to convert from the 5000ohms expected at the end of the EFHW to 50ohms on the link coupling. I started with 30 turns of hookup wire on the secondary and 3 turns on the primary. This resulted an inductance in the tank circuit of 11uh and a small capacitance needed to resonate the circuit at 7020Khz of about 47pf (Xc/Xl=485ohms). I realized the capacitance by using a piece of RG58/U coax connected in parallel with the inductor. This having a capacitance of 28.8pf/ft. Thus the coax piece was about a 1.6ft long. The circuit was very unstable. Small amounts of capacitive coupling resulted in a major shift in the resonant frequency.

I experimented with a number of turns ratios and discovered that any combination resulting on a coax length of less than about 2 feet was unstable due to capacitive coupling effects from surrounding objects including my hand.

I reduced the turns down to 18Turns on the Secondary (4uh). I was able to get a good match using 2turns on the link coil and by using some stiff stove type wire, insulated which I bought from Builders Hardware. This required a coax length of 4 feet (~115pf). I was able to get a 1:1 SWR using a 4.7K resistor as the terminating impedance on the bench. The circuit was stable and showed no capacitive or inductive coupling effects to nearby objects. This shows approximate values of 4uH for the inductor and 115pf for the coax capacitor to yield a resonant frequency of 7020KHz.

When I connected the EFHW wire to the coupler I was able to obtain the best match at 6.8Mhz by driving the system with my MFJ259B antenna analyzer. By tuning the analyzer up to 7020Khz I noticed a change in SWR to about 1.6:1. Since I had already resonated the tank circuit at 7020KHz against a pure 4.7Kohm resistor I was hesitant to tweek the capacitor by trimming it shorter. Instead I trimmed about 6 inches off the EFHW wire and was pleased to see a 1:1 SWR match on 7020Khz. This confirmed my feeling that the wire was showing a 5000 ohm pure resistance at resonance (in spite of being pulled through a tree at one end!). In looking at the write up on the web by AA5TB http://www.aa5tb.com/efha.html I noted that an EFHW will exhibit a 5000 ohm resistance with a 0.45 wavelength counterpoise. I must conclude that the mass of the antenna analyzer and the short coax connecting to the analyzer of a few feet is sufficient to couple the system to ground and simulating a quarter wave counterpoise (I really don't know however). I also experimented by linking the 'cold' end of the secondary to the 'cold' end of the link coupling. I noted with interest that this had no effect at all on the SWR of 1:1 at 7020Khz. The system is quite stable.

Much more experimentation is warranted on this interesting antenna. Especially for that case where the rig may not be near the ground or coupled to ground in some way. In this case a short counterpoise will likely be needed. Also the effects of different deployments and wire length variations on SWR is room for much experimentation.

Experiment to try in future. Orient the rig and coupler well above the ground in the air so that there is no (little) coupling to ground. Then measure the SWR variation. Then attach a 0.05 wavelength counterpoise to the 'cold' end of the secondary. In theory the antenna end impedance should drop to about 1800 ohms. This would require a 6:1 turns ratio. In other words another turn on the link coil to bring it to 3 turns and 50ohms match. According to the AA5TB graph, at this counterpoise length the reactance should be zero at the end of the wire and the system should be stable.

Observation. The practical point of this type of experiment would be to see if it is possible to use a coupler in the field with fixed components. This would greatly simplify the deployment to a blackbox that requires no tweeking. Furthermore....could this coupler be integrated into a single box along with the transmitter and receiver. In other words how critical is the length of coax between the coupler and the rig. Is this acting as a counterpoise in reality?

Bandwidth. The bandwidth of my system at the 2:1SWR points is approximately 145Khz. What is the effect on Q of different component uses and combinations (thicker wire and air variables)? For me 140KHz bandwidth is more than enough. Referring to a previous blog where I define the scope of my field operation I would trade bandwidth for a better Q since this (I think) would mean less resistive loss in the coupler (I think). Of course a very high Q would mean a less stable coupler from the point of view of field deployment and the effect of nearby objects on the system.

Bench Test using a 4.7Kohm terminating load and coax capacitor

3 Hornbill St Cedar Lakes, Fourways, Gauteng

I was very pleased with the reports I received on the Saturday AWA net on May 4th. I deployed an 'inverted U' type EFHW in my tiny backyard. I guess it was too cold to venture out into the park. Necessity being the mother of invention, I had to check into the net. The pic below shows the configuration. I used my field coupler for this net, however I have also built a large coupler using a T-200-2. I will write up these experiments in a separate blog.

I have also made up a new and vastly improved launcher. See the attached pic. This consists of 3 fishing weights on a nice 30ft piece of slippery yellow cord. I have also tied a knot about 3 ft from the fishing weights. This allows me to get a grip of the thin cord between my fingers during the launch. The launch weight is about perfect for a 20ft high launch and the cord slides easily through the target tree and to the ground. The launcher is compact and thus easy to store in a small backpack.

Extreme Rapid Deployment Operation 

Revision 1

Herewith is a description of my requirements for an Extreme Rapid Deployment Field Operation.

You will observe that this is highly optimized to perform one type of operation only.

1. Rapidly deploy in a park after a short walk during the daytime. Target under 4 minutes deployment time.
2. Ultra Ultra simplicity.
3. Make at least one QSO within an area the size of South Africa on CW, 7020Khz (in SA) or 7030Khz in the USA during one hour, NVIS operation.
4. Utilize a very simple direct conversion receiver with minimum components and moderate sensitivity and selectivity of 1.2Khz or even greater will be acceptable. To be quantified.
5. Utilize a very simple CW transmitter coupled to an EFHW coupler and able to transmit 5watts, 500milliwatts or 50milliwatts output power. Measure SWR exceeding 1.5:1.
6. Fully understand every voltage, resistor, capacitor, transistor in the circuit and the performance of the EFHW antenna. In other words 100% homebrew.
7. Carry 8 separate items in a small backpack.
1. A rugged lightweight enclosure consisting of all homebrew circuitry. TX,RX,SWR,ATT,EFHW Coupler, 12V BATT. I don't know the enclosure size yet.
2. Earplugs
3. Handkey (extreme lightweight using a microswitch).
4. EFHW 66foot 18awg wire with plug on one end.
5. 30 foot slippery cord with fishing weight on one end. Launch 15ft max into a tree.
6. Miniature notebook and pen
7. Small bottle water
8. Leatherman
8. Obtain a signal report at 5Watts, 500mWatts and 50mWatts
9. Give an accurate signal report.
10. Do a bit of ragchew
11. Confirmation that SWR is less than 1.5:1 using simple LED indicator.
12. Battery capacity for one hour operation at 5 watts 25% duty cycle. (this is an interesting part). This is 100mAHr capacity. 

The battery size and type needs some experimentation. I have used fancy technology LiPo's in the past. However they are a pain to charge etc and are expensive. So I reckon 9 AAA type batteries would be a good place to start. Will do some research here. 

In every case in my experience here in Joburg I have found a tree with easy deployment to 15foot. Then I have found another tree to sit under, about 50feet away with an inverted L type config. The efhw works excellently. I received a 509 report from Pilanesberg over the weekend from a station in Namaqualand Springbok using 2 watts.  ZS6JBJ gave me a 559 report from Witbank using 200milliwatts on my K2. 

My K2 is a joke for this operation. Talk about overkill!

I now am gathering the parts to build the TX/RX and attenuators. More to follow. 

Manyane Campground, Pilanesberg game reserve

Berry looking happy :)

On April 30th Berry and I travelled to Manyane for a 2 night visit over the May day holiday. A very pleasant time was spent there. From a radio perspective, I was able to set up a very good EFHW into a tree outside our campsite and about 30ft agl. I then formed an inverted L configuration with the vertical section about 12ft long and connected in the usual way to my tuner. I was QRV at about 10:00 local and received a 599/QSB report from ZS4SF in Welkom. I could tell that the antenna was working particularly fb. I ran 2watts. After that QSO with Monk I called CQ for about 10 minutes with no takers. I then tuned up to 7070Khz and was able to enjoy a number of QSO's on SSB and running 5 watts. It is clear that making contact on Phone here in SA is a lot easier than CW. On Phone I chatted to ZS6JSE, John in Carolina 55/56, ZS6BTY, Vince in Pretoria 58/59. Vince was operating QRP 5 watts. Some QSB. He was using an extended double Zepp for 40m.I then talked to ZS6GQ in Alberton, Sid who was running 100W into an inverted Vee, 56+QSB.

Later in the afternoon at 13:12 local, I worked ZS3AOR, Pieter in Springbok Namaqualand on 7020Khz CW. I was amazed that he copied me. My signal report was 339. Unfortunately I did not copy his antenna or power. I then had an excellent QSO with John ZS6JBJ in Witbank . This was fun. John gave me a 599+20db report on my 2 watt signal. I then reduced my power to 1Watt and received a 599 report. At 500mW he reported 579 and at 200mW he reported 559 with QSB. This was proof that my EFHW was working exceptionally well and that the condx at this time were favorable. Towards the end of the QSO I could hear the band going long and DX music and pirate stations were heard. Overall I was very pleased with the result.

Observations:

1. If I can QSO Namaqualand on 2 watts in mid afternoon then my field setup has day communications Nationwide.
2. 200milliwatts is probably all that is needed for essential communications around Gauteng, NW province and Mapumulanga provinces.

'Room with a view'

The beloved Hornbill

South African Moonshine!