[R-390] antennas

[John Franke]

I thought the quarter turn connector is a C connector.  It mates with my C 
adapters.

John  WA4WDL

----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <R-390 at mailman.qth.net>
Sent: Wednesday, October 17, 2007 6:58 PM
Subject: Re: [R-390] antennas


> Rasputin,
>
> Once upon a time the Army run an antenna coupler between the antenna and 
> as
> many as eight receivers. The coax between the fan out antenna coupler and 
> the
> receiver was stock 50 Ohm RG 8 coax. The receivers had a right angle 
> adapter
> that grounded one side of the twin ax connector and coupled the other pin 
> to the
> center conductor of a D connector. the same connector that is on the un
> balanced input. It is better to feed the receiver single ended into the 
> balanced
> input than to feed the receiver into the un balanced input. The D is a 1/4 
> turn
> disconnect like the smaller BNC. These adapters are about $23.00 these 
> days.
>
> The Signal Corp. knew what freq to tune the receiver to and when to 
> listen.
> They were not doing weak signal work. they could afford to run a whip into 
> the
> un balanced input and still get a receivable signal.
>
>
> The input impedance of the balanced input is over 250 ohm across most of 
> the
> receiver range. The low antenna Z into the higher receiver load gets you a
> better voltage on the first RF grid. The vacuum tube being a grid voltage
> controlled device. This works well. Sort of like using a high input 
> impedance vacuum
> tube volt meter to measure voltage with. The high Z gets you a better more
> accurate reproduction of the signal being sensed.
>
> If you pick a core material that functions in the HF range of interest, 
> you
> can easily make up a good transformer that will match the un balanced long 
> wire
> to the balanced input. Think 250 Z balanced receiver input and then do a 
> 1:1
> 2:1 or 3:1 core winding to match what you expect on the antenna. Think 
> 2000 Z
> on the end of a random long wire and try 9:1 for the balun. Go for about 5 
> to
> 15 turns on the receiver side of the transformer.
>
> Most core material will not cover the whole HF band. This is good as it 
> acts
> as a band pass filter stage between the antenna and the receiver. You can
> filter out the AM broad cast band this way. However to get a good filter 
> you need
> a short length of twin ax and a twin ax connector for the receiver. You 
> then
> couple the twin ax into a separate grounded metal can. Inside the can you 
> feed
> the center conductors from the balun coil. After you filter the AM or 
> other
> unwanted signal off the feed line you need to shield the rest of the feed 
> line
> into the receiver to keep the same rejected signal from coupling back into 
> the
> feed line after the filter point.  Some Fellows have built nice tight 
> boxes
> right at the antenna relay to do this job. Some have used a separate box 
> on a
> length of twin ax. Some have tried to do this decoupling and filtering, 
> and not
> getting a good shield have observed poor results and just give up on the
> problem.
>
> See the web page from the other mail. The coupling cap works real well and
> keeps the static voltage from the antenna off the receiver.
>
> This single ended approach into the balanced input worked for the military
> for years and on most of the receivers.
>
> Roger AI4NI   </HTML>
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