27 March 2013

EFHW field test idea & tuning an EFHW

I am recording here an email I received from Monk ZS4SF who is in Welkom and a keen cw op on 40meters and my response:

"Hello Dick ,
Thanks  for  the  mail.  Thought  I  would let you know that I will be monitoring  the freq. on 40 meters on Sat.and Sun. mornings. I have put together  a  small fm tx that I plug into the hf rec.( ear socket )and tx  into  the  house.  RX on a portable fm receiver. On Sundays I will monitor only until 1 PM.
Okay  on  the efhw antenna, the day we worked I was using full power (100  watts,  but  even with 1 watt the chaps in Durban and Jhb give me over  the  9  reports.  I  would  love  to  build a qrp rig that I can attenuate  down  to 50Mw and less. I have the circuit for it but can't obtain the toroids in this country.
Do  you  cut  your  efhw  for the lowest freq. and then tune the other bands  ?  This  I  have  not tried.  In your blog you mention that you touch the rig and observe the swr to see if you have any rf around. I  have found that this does not really work.You must grab the coax at a  voltage  point. I have my wife run her hand up the coax while I key and look at the meter. With Qrp I don't think it matters.
CU on the band

Hi Monk,
many thanks for the response and very excellent on the monitoring project.
Yes I heard you qso with Andy ZS6ADY last sunday morning on this initiative to monitor the 7020Khz freq. I can tell you that I don't have a set up here at the apartment (yet) but any chance I can get to take a walk in the park and set up I will definitely do it. 
I am very interested in trying some experiments using the efhw. I reckon it may be very value-add to those interested in field operations. Here is the idea I had in a nutshell:
I would set up my ZS6TJS 7metre high mast at suitable locations around Region 6 in the clear. I would then erect an efhw on 7020KHz to the full mast height (7m) with the wire broadside to the receiving station (for example ZS4SF...) then transmit at 5W, 500mW and 50mW cw signal. Obtain signal reports. I would then lower the efhw to 4m and obtain signal reports. Then lower it to 2m and obtain signal reports for the 3 power levels. Then I would orient the efhw so that it is pointing straight at the receiving station and then repeat the tests.
Repeat the above tests a few times during the day. Say 7am, 11am, 2pm and 4pm.   Then if it yields interesting results I could try different locations and path lengths over the course of time.
The objective of such a test would be to see what the limits on height, power per time of day/year on 40meters are for field stations in region 6 South Africa. Of course one could go crazy and do it for different frequencies. Night tests would be really interesting..... One could also try different configurations. I find that invariably here I am sitting under a tree and have the wire strung across to another tree. So I am mostly in practice using an inverted L type config. This seems to work very well.
Do you think this type of experiment would be interesting? Perhaps it has all been done before? 
Ok on the need for a small QRP rig and agreed 100% on the need for 50mW for the reasons shown above. Please let me know what parts you need? I will be able to get them from W Land for you no problem. I have people coming across from the USA from time to time who can carry a small bag of stuff. In fact I may also join you and build one of these rigs. Must have a built in attenuator. Can be rockbound on 7020KHz for this purpose. Let me know and I will work with you on this one.
Interesting on the efhw and the voltage point. In practice I really only use a very short piece of coax to connect the tuner to the rig. I suppose I had assumed that by touching anywhere there I was in fact at a voltage point. I will definitely try to connect a long piece of coax and see the effect of touching at different points.
Here is the way I tune the efhw if I really want to do it 'properly and according to the book'. First I connect a 5K resistor across the tuner (link coupled parallel circuit). I then tweek the capacitor to minimum swr dip. Then I disconnect the 5K resistor and connect the efhw to the tuner and tweek the length of the wire until I achieve minimum SWR.  
In practice I have realized that there are several flaws with this method. The most obvious is that the literature stated impedance of 5Kohms at the end of the wire is not the case in practice since the lower-end is likely coupling to surrounding objects.... I have experimented with raising and lowering the fed-end closer and nearer the ground and have noticed significant differences in the resonant point etc. This is why I am a bit curious regarding the use of a fixed component tuner in the field such as the PAR END FEDZ. How much does the SWR vary? There is huge room for experimentation in this area.  In practice I simply sling a 66 foot piece of wire into a nearby tree and tweek the parallel tuned circuit for minimum SWR. Seems to work FB. Seems to work far better here than at my home QTH in the USA where the soil is clay and verticals are not popular. There I get reports of deep QSB and a few s points inferior to a field dipole. Here in SA the signal level seems to hold much steadier. Why is this?
Yes as you mention below I have tried to tune a 66 foot piece of wire for operation on 20m and 15m and 10m using the same tuner. Obviously now it is not behaving as an efhw. I am able to get a resonant point on those bands. However not surprisingly the SWR is now not 1:1 as per 40m since the turns ratio for the link coupling has not been changed. I have not recorded the SWR but I am able to use my internal K2 Elecraft ATU to obtain a 1:1. What I have not tried is to use an efhw on these higher bands with the same tuner. I imagine it would work but would not be matched optimally.
  Many thanks Monk for your comments and questions. I can tell that this subject is of interest to you. Thanks for setting up the FM TX. Sounds like fun! I will be sure to be one of the regular call ins. 
73 Dick 
The 5K calibration resistor can be seen near the blue efhw wire

A simple parallel tuned circuit designed to load into 5kohms

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