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" <rich...@karlquist.com> To: "Pete N4ZR N4ZR" <pete.n...@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 _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector