I agree with Rick, all of the CATV RG-6 I've tested is much more
than 75 ohms, in one case it was 95 ohms! Depending on length,
this can cause significant unwanted impedance transformation.
73
Frank
W3LPL
----- Original Message -----
From: "Richard" <richard@karlquist.com>
To: "Pete N4ZR N4ZR" <pete.n4zr@gmail.com>, "topband" <Topband@contesting.com>
Sent: Sunday, April 2, 2023 12:15:46 PM
Subject: Re: Topband: 2-element receiving arrays
On 3/30/2023 5:54 AM, Pete Smith N4ZR wrote:
> Thinking ahead to next winter on 160, I'm interested in replacing my
> K9AY Loop with a 2-vertical phased array. I'd like to homebrew the
> antennas and just buy or build the remote control unit for the shack.
> I'm looking for sources of components (antenna-located preamps and an
> in-shack controller), and would prefer not to completely homebrew them,
> but the prices at the usual suspects are awfully high. Any ideas?
>
> I have pretty reasonably-priced access to 25 and 31-foot fiberglass
> poles (used for wind-socks by model airplane enthusiasts). I'm thinking
> that one relatively low-cost approach might be to attach, say, #14 wire
> to the poles, with preamps at the base, but wonder if there is a
> downside to using such small-diameter antenna elements rather than 1 or
> 1.5 inch tubing? Alternatively, are clones of the DX Engineering 8'
> short verticals with preamps a good alternative?
>
Answering your question about the small diameter antenna element:
I happen to use 2 inch diameter 30 foot long irrigation pipe "because I
can" meaning I got a good deal on a bunch of it used. The pipes have
4 way guying (much easier to tilt up than 3 way) and I have four
"umbrella wires" that are 21 feet long and are zip tied to the guy ropes
which go to ground anchors 30 feet out from the pipe. This puts the
height of the umbrella wire tips at 15 feet. The umbrella wires are
highly recommended to give additional gain. But I strongly suspect
that they would also mitigate any issues with using small diameter
wire in place of 2" pipe. If you only use the wire without the
umbrella wires, you can easily calculate the reduced capacitance
due to the small diameter using the usual formula. Fortunately,
the function is logarithmic not proportional to diameter to length
ratio. Voltage gain is basically proportional to antenna capacitance.
I have a 75 to 300 ohm transformer right at the feedpoint (made with
the usual binocular cores). This gives me additional gain vs feeding
directly from 75 ohms. It works against four 30 foot long radials on
top of the ground. I have found that this configuration gives
plenty of signal without any preamp besides the one built into the
radio. I live only 6 miles from a 50 kW AM BCB station, so preamps
are a problem here. The DX engineering preamp with the 8 foot CB
whip gets creamed by the BCB QRM, and there is no way to protect
it by inserting a BCB reject filter, AFAIK.
I put the combiner midway between the verticals and have a couple
of relays to select 0 or 180 degrees or omni. I will strongly second
the other posters about NOT putting DC on the coax ("too clever
by half"). I happen to have a bunch of CAT5 cable on hand and use
that for the relay coil drive. The "controller" in the shack is
just a few toggle switches.
A possible idea to explore is to use LATCHING relays at the antenna,
and then it is OK to run DC over the coax momentarily. Amazingly,
I haven't heard of anyone trying this (including me).
For the connections from the antennas to the combiner and for the
coaxial delay line at the combiner, it is worth considering using
RG59 coax with a true copper center conductor. Every sample
of RG6 with the copper plated center conductor varied considerably
from 75 ohms on top band and was frequency dependent. It you
do use it for the delay line, measure its time delay at your
operating frequency. Many delay measurement techniques rely on
measuring delay at some "convenient" frequency and assuming
it is "non-dispersive". Wrong.
Anyway, this is what works for me; YMMV.
73
Rick N6RK
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