[R-390] antennas

[Flowertime01 at wmconnect.com]

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>