Looking at this CB Radio antenna you may think I am crazy to use it for local and Skip!
BUT it is far better in operation than it looks!
I should make it clear at the start that my wife when I first met her was oblivious to all the various CB antennas I strung up or mounted on lattice towers all over her established home property in the outer suburbs. A massive tree in the back yard had many HF wire antennas and the house supported a tower with a HB9 Swiss Quad with a vertical above it that I was able to work the world on in the eighties.
Until we / she decided we should move to an new off the plan apartment in the inner city 10 years later. Well considering I made great friends with the owner builder of the apartment complex, it was no trouble to put three runs of heliax through the walls up to the flat iron roof four stories high from my designated radio room on the second floor. A high gain 12db 476MHz UHF CB coaxial colinear worked awesome.
But there was now way I could run the Swiss Quad and Vertical up there so another strategy was needed.
I had always been fascinated by unusual antennas across HF bands and the DDRR was always of interest to me. So simple to build just like a Quarter Wave Nine Foot mobile whip with a ground plane, but the added advantage of DC grounded, so low QRN noise! Also the height of the vertical part only needs to be twelve inches on 27MHz! The total length is a 1/4 Wave of the center operating frequency. The circular top hat is only a yard in diameter ending about eight inches from the vertical part giving a tunable capacitance.
The polarisation is both horizontal and vertical it seems.
Note that this is my redesign of the original DDRR that was designed for warships by the inventor. This has no tunable physical component of a capacitor, just the gap at the end of the top hat. This gives very high ‘Q’ of only five CB Channels each side of set resonant frequency which I tune with a Nano VNA H5 to 27.355. This usually occurs at about eight inches from the vertical part around the top hat.
As most SSB action happens between 30-40 it is fine and the VSWR less than 1.5:1, but i do peak with a tuner inline if I want to go further up or down the channels. It could be modified with a variable capacitor if you wanted to go to all the trouble for more band width.
From the Apartment after a few years we moved to a Townhouse on a hill five kilometers from the city, happily as someone had recently mounted airconditioning on the apartment roof five yards from my antennas! Unhappily I had lost a fortunes worth of heliax!
So had to go back to a nine foot stainless whip on top of a water air cooling metal box on top of the tile gable roof that worked ok because of the high location and the water plumbing earth. By now my wife made it clear that no obtrusive antennas would be welcome like they were on her older house.
So moving again to a Bayside town between two major cities across the water into a two storey house with a flat iron roof and no visible antennas allowed. So the DDRR seemed to be the only answer to my dilemma of wife approval. With only twelve inches height the DDRR was invisible, and if one is good two must be better! So I co-phased two and worked a lot of DX interstate and over seas.
Ten years later in mid 2021 we have moved again to a tree change home in the country over looking a large body of water with a gabled iron roof.
Luckily I have a flat iron roof on the garage rear shed I have turned into my radio shack so perfect for DDRR’s!
But the original DDRR’s I had were made of one inch copper stranded wire that was extremely heavy and did not make it onto the removal truck. But I had bought some copper tube for the purpose many years ago at a garage sale cheap so have what I need to build two more.
This is the first DDRR and I had to have a way to mount it on the iron roof easily so found an old rusted out CB base that with a bit of modification earthed out on the base and the roof as the radiating element has to be DC grounded for the low noise operation.
-1.1:1 SWR @ 56 Ohms impedance on 27.305 MHz – Prefered setup with Antenna Analyser or good SWR Meter in actual location.
D – USE Copper/Aluminium Tube or Coax – Heliax or EVEN RG213 would work I think! (But lower band width, maybe five channels with low SWR)
SP – 12 ” high
L – 108″ long Copper Tube or Coax cable
G – 8″ – 9″ gap from upright to radiator end. NO CAPACITOR!
FP – 8 – 9″ tap from upright along radiator. TO BE TESTED for LOWEST SWR on center of most used channels!
As you can see the NanoVNA is reporting a 1.05 VSWR on 27.390 MHz on the new DDRR!