Hello Mark;
I use chokes at all my vertical feedpoints. In a four square there is
significant VSWR (which is expected) when feeding verticals with 75 ohm coax
as they have complex base impedances caused by mutual coupling amongst the
verticals and 300 KHz of frequency coverage (for 80 meters).
I use DXE RG-11 foam for phasing/feedlines in my four square arrays (six of
them here) and the total length of the coax is considered (end to end of the
PL-259s). For 80 meters I set the DXE RG-11 phasing lines to 3.650 MHz. I
simply use forty #31 ferrite beads over the coax - then heat shrink tubing
with glue at the vertical feedpoint end. The CMI of this choke is over 5000
ohms at 3.650 MHz. With this setup (Comtek box) the dump load power is
better than -26dB at 3.650 MHz - which is excellent. -26 dB equates to 3.75
watts in the dump load - if 1500 watts were applied directly to the box.
It is critical (for best performance) that EACH vertical in the array is at
the exact same frequency at resonance (X=+/- j0) as the other verticals. The
resonant frequency for the verticals in a four square array is lower than
the target lowest dump frequency and varies by what is used for vertical
elements. Skinny wire verticals have less frequency deviation (relative to
3.65 MHz) than FAT tower verticals which must be tuned below 3.5 MHz for
lowest Comtek (or DXE) dump power at 3.65 MHz. Tuning verticals must be done
by floating three verticals and adjusting one - at a time.
Good Luck and 73
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Michael Tope
Sent: Monday, December 2, 2024 4:41 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Adding an RF choke
Mark,
What is the impedance of the feed lines that connect your array
controller/hybrid to the individual elements?
If, for instance, those feed lines are 75 ohms, then you should use 75
ohm coax for the chokes and then shorten the feed lines by
18*Vfeed/Vchoke inches, where Vfeed is the velocity factor of the coax
used for the feed lines and Vchoke is the velocity factor of the coax
used to make the choke. If the choke impedance is different than the
element feed line impedance, then you are going to upset to some extent
the impedance transformation provided by the feed line. The higher in
frequency the greater the impact (e.g. 18 inches at 1.8 MHz is much less
of an impact than 18 inches at 7 MHz).
73, Mike W4EF..................
On 12/2/2024 8:26 AM, Mpridesti via TowerTalk wrote:
> Building a set of phased verticals and the phasing lines are critical (of
course) so my question here: by adding 18 inches of cable length (choke
length) does the phasing line length need to be shortened by 18 inches?
>
> Or should I consider the choke as part of the antenna and not make any
adjustments?
>
> Regards,
>
> Mark, K1RX
>
>
>
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