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DDRR antenna
2

DDRR antenna

DDRR antenna

(OP)
I wonder if anyone can tell me how to design a DDRR antenna for 150 MHZ transmit and 137 MHZ receive (Orbcomm. I need a low profile antenna that can be mounted on a truck roof. Thanks

RE: DDRR antenna

It is about the same as a small magnetic loop which is smaller than a quarterwave in circumfence and needs a special high voltage tuning capacitor.

I recommend to build the halfwave version which is higher in efficiency, slightly larger diameter, slightly higher and a much wider bandwidth. The effort is even lower, just two closed rings or one closed ring over a groundplane. You do not need the special high voltage tuning capacitor!

RE: DDRR antenna

(OP)
HF Mobile

Thanks for the input. As you suggested, I built a 1/2 wave DDRR Circumference is 37 1/2" using 1/4" copper tubing, and a solid copper ground plane. I could get it to work beautifully at 75MHZ where it is 1/4 wave length, but can't get it to work at 150 MHZ at all very high impedance and SWR. I tried a verticle section as short as 2" to as large as 4". Do you or anyone else have any suggestions?

RE: DDRR antenna

Hi Mr. "ZV";
You might want to examine an antenna offered by Antenex Inc. located in the Chicago, Ill. area. www.antenex.com
They offer a low profile (3.5" hgt.) device they call the "Phantom" which installs on the standard Motorola 3/4" mount.
I have used several at UHF and while the manufacturer hypes them as providing 3dBm (their own measure) of gain, they behave at least as well as the antennas they replaced.
Another possibility, which I would NOT recommend, is an offering from ASP (Antenna Specialists Inc.) which is in the form of an elongated radome (14" long 3" high 1.5" wide).
These are relatively narrow banded and rather complex internally. I've seen quite a few of them fail due to fractured internal solder joints. They are also quite expensive at better than 100.00 US each. BTW, the Phantoms replaced ASP's UHF version of the antenna mentioned above.
Given that you are only looking for a hi-band antenna, the same Antenex Inc. offers a heavy duty quarter wave antenna, the A150S which stands about 18" tall. If your primary concern is striking the occasional tree branch these will survive quite nicely.
I have several customers who will allow me to install only that particular model on their vehicles which are 10.5' high schoolbusses, a severe test of any form of antenna.
Stan,
Stanley Communications

RE: DDRR antenna

Getting a DDRR antenna to operate at two frequencies so far seperated will be a tough nut to crack.  The DDRR antenna is quite narrow band.

The answer to "Broadbanding" the DDRR would be to increase the diameter of the radiator.  I'm not too sure that it would be practical to increase the diameter of the tubing to the point where you could get 13 MHz bandwidth.  At the frequencies you quote the difference in wavelength is nearly 19 CM.

There are several on-line articles about the DDRR antenna.  A good engineering library should have the original article in Electronics magazine.

I've built a few DDRR antennas down through the years including one for the 27 MHz CB band. which my father mounted on the roof of his '53 Chevy Pick-up.  That one was out of 5/8" copper.  The 2:1 VSWR bandwidth was three CB channels wide.  I put a small Vacuum Variable in a box and extended the shaft through the roof to a spinner knob so that all 23 channels were usable (YES, it was THAT long ago...I'm not the OLD Soldering Gunslinger for nothing)IIRC, this was around 1965 since I was looking (Drooling) at the '65 Mustang Convertible.

Now that I have access to a really grand Network Analyzer, I ought to build one for 40-meters and maybe write a paper on it for SBE points...I wonder what the homeowners assn would say about that antenna in my back yard?

I remain,
The Old Soldering Gunslinger

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