2012/04/25 at 10:23 AM #110Greg 43AX005Member
DDRR Disconuity Dsomething Ring Radiator -GREAT STEALTH
by Bellhopper » Wed Sep 15, 2010 1:24 pm
Well I finally have built one after talking about it for years!
I guess necessity is the mother of invention, or my wife is at least.
“GET ALL THAT CRAP DOWN OFF THE ROOF!!”
Well you know the story about a woman scorned and all….
Anyway thought the only alternative to massive 27MHZ verticals that have omnidirectional vertical polarisation and dont stick up in the air is the DDRR!
Had picked up a roll (only just!) of 20mm diameter stranded copper wire that I thought would come in handy years ago.
So cut a length at 106?” to make a quarter wave and got a post bracket at Mitre 10 to use as the verticle post so I could clamp it to the stramit steel roof with a G clamp. Cut it off at 12″ long and put about four inches of bared copper cable inside and then bent it so it would create the “halo” on top of the 12″ verticle bit.
As I wanted to test run it first in the garage to work out the feed points and lengths etc I cut another length to use as a counterpoise on the concrete garage floor.
I balanced the curled top radiator on top of some cardboard boxes to keep the radiator the same 12″ distance above the copper ring ground plane on the floor earthed to the bottom of the 12″ vertical radiator.
AS the plastic cover on the cable was industrial strength I hammered in a few nails into the copper core as feed points to test for best match on RX. I did not want to TX on the floor in the garage.
After hooking up the temporary 50ohm RG58 feedline to the first tap point about 8 inches from the vertical and the coax braid to the ground plane copper ring right below it, the old Tram D201 squarked out some audio!
The RX was down a bit on the usual quarter wave on the tin roof right above it, but encouraging to say the least.
Probably 2 s points down on a known source 50 klms away. And this was under the tin roof on the garage floor!
I fooled around with it moving the end of the cable in the circle closer and away from the vertical and the difference improved by 1 S Point in the right spot about 10″ away!
I should say right about now that I have built a few HF antennas with slightly more attention to technical measurements than I did with this one! I did read some info on it on the web and roughly rigged it up similar.
I must admit I was surprised at the immediate positive results!
Because it looked like it was going to work I thought that I would put it up on the garage roof connected to the RG213 I had used on the 1/4 w vertical. I made up an adapter out of a female/female so239 and a short pice of RG58 stripped and sepparated so the braid and core were able to connect to the top and bottom feed points.
The centre core went to the nail in the top ring radiator and the braid went straight below it to the tin roof ground plane. The two old cardboard boxes were used once again to space the top ring away from the tin roof ground plane.
Will need to be replaced with a decent insulator spacer soon as they wont last long in the rain!
I “G” clamped the vertical bit at the bottom to the roof which obvoiusly shorts out the whole thing and contributes to the very low QRN advantage factor of this configuration.
Getting down off the roof I was looking forward to hearing and working all those Melbourne BOOM stations way down here on the coast 50 klms away.
WELL! What a disappointment! I could not hear a thing! Just maybe Squidley in the background mud although there was virtually no noise at all!
To cut a long story short, after trying to match it in with my Ham Matcher and getting an acceptable SWR but not hearing anything, I realised I was still on the original test cable lying on the garage floor!!!!
I had forgotten to swap over to the RG213 cable I had used on the 1/4 wave that was now on the DDRR!
Haleluelyah or something! As soon as I hooked up the DDRR on the roof the signals came flooding in on RX.
After a bit of matching I was able to TX on 27.355 at 1:1 with about 15W from the CPI BC2000.
Got some good reports from Melbourne and I am sure it was working better than the 1/4 wave by 1-2 S Points!
BTW I always had to SWR match the 1/4 wave from about 3:1 down as it did not like to sit on the flat garage tin roof.
So after all that I know have to do some fine tuning but it can only get better I am sure.
The resonant point is down at 26.300 so it is a bit long at the moment, but I now realise that I measured thecoppper wire at a 1/4 wave and added about 8″ with the vertical metal mounting post!
So i suspect if i cut about 8″ off the end of the ring and then move it in back about 10″ from the vertical it should resonate around mid 27mHz.
That will be my next trick and I will report back soon on the outcome if anyone is interested?
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