Low loss, high efficiency dipole feeder...

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Low loss, high efficiency dipole feeder...

Post  G7VGG on Tue May 26, 2009 5:48 pm

Up till now I have been using a cheapo open line dipole feeder for HF, which I picked up for £5 at Martin Lynch and sons and I have been running 15m of 300 Ohm balanced ladder line straight from the dipole feeder to the back of my ATU, and the results have been very good, considering how low my Dipole is.
However, after purchasing a field strength meter I noticed a worrying amount of RF in the shack when TXing on HF.
I recently built a homebrew base mike and on all HF bands except 80m (and maybe 160m too?) the RF in the shack is finding its way into the mike and causing distorted audio...Use of Ferrites helped but did'nt cure the problem.
So I decided to try an idea I found in the ARRL antenna book...75 Ohm "shielded" balanced line. The idea is to use 4 identical lengths of RG-58 coax in parallel to transform the normal 300 Ohm impedance of balanced line down to the 75 Ohm dipole feedpoint impedance and the braids of the coax are used to sheild the balanced line from reactance effects which occur when open line is run near metal objects and also against static pickup and pickup of man-made noise. The braids are connected together at both ends but at the shack end they are run to ground and they are left floating at the antenna end.
RG-58 coax weighs more than 300 Ohm balanced line and 4 lengths of RG-58 weigh considerably more and so I was'nt sure my cheapo dipole feeder was up to the job...The cable clamp on the feeder would'nt be wide enough in any case so I decided to make my own heavy duty dipole feeder with the capacity to clamp at least 4 side by side lengths on RG-58.
My chosen material for the feeder was 8mm thick white foam uPVC sheet. Foam uPVC is an excellent material for building antenna projects. Its strong, very light, its completely water resistant, its unaffected by exposure to UV light and its easy to cut, shape and glue together.
Its also an excellent RF insulator with virtually no RF absorbtion at all, so its great for making antenna support brackets...Best of all I found a load of it for free!
The feeder required 3 identical Foam uPVC layers to be cut out of the sheet, which were marked out with a template and cut out with a bandsaw.
The middle layer is cut in such as way that when clamped between the other two layers it forms a water resistant box which houses the dipole terminals, cable clamp and strain relief bolts.
It is glued onto the base layer in three pieces.
This is a great improvement over the cheapo dipole feeder I used before which has totally exposed terminals.
To ensure a positive connection to the terminals I made a large copper solder tag for each terminal , fabricated from some 15mm copper tube. The tags have two bent upward tabs on them, at roughly 90 degrees to each other which are drilled to take the centres of the coax and the end of the antenna wire.
The solder tags are clamped to the terminals by brass washers and brass nuts inside the feeder. The terminals themselves are a couple of 19mm long M5 cheesehead Brass screws.
At the shack end of the new balanced line I made up another two Copper solder tags and soldered two of the coax centres to one and two to the other. Then I bolted these to my BL-40X 1:1 balun and ran a short length of RG-213 coax from the balun to my ATU.
Now the antenna is self resonant on 80m with an SWR of about 1.8 across most of the band without the ATU, which is more than acceptable in my opinion.
After testing with my RF-strength meter whilst TX'ing on the HF bands I can confirm that the RF in my shack has been reduced to totally insignificant levels and I no longer have any RF pickup on my base mikes audio.
Furthermore, noise on each band has been significantly attenuated whilst the RX'd signal strengths of other Hams is no less than before or in some cases considerably greater than before!

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