20 May 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!

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.

No comments:

Post a Comment