They exist for military applications but use resistive components to help achieve a good match to the radio. One example would be the 13” rubber duck for the Thales MBITR (PRC-148) radio. It covers 30-512MHz with a very reasonable match but they warn the range is seriously diminished from 30-90MHz where the gain of this antenna is around minus 30dBi. Otherwise it works very well everywhere else. Another example is the Shakespeare 3512 series http://shakespeare-military.com/catalog/section3512.pdf which is for mobile use. On some of the mobile antennas it’s common to use around a 3dB attenuator at the input of the antenna to keep a reasonable match. A 3dB pad will give a 6dB return loss, which means the radio will see around a 3:1 VSWR even with the whip part of the antenna broken off. The down side is half your power on transmit and receive is lost not counting other deficiencies in the process.
Actully i have three bands 30-88, 136-174 and 403-470MHz.
So initially i design my normal mode helix length by taking lowest band(30-88).
that is at fc=59MHz, lamda/4 equals 1271mm(N=66, pitch=14.5).
Than i make it helix having 6 mm dia.
But this helix will cover only one band(30-88). What about the other bands..???
Somewhere i have read in papers that, others bands can be cover by varying dia or pitch angle of the helix...but unable to find much details...and what about the matching network of this antenna.
SO If anyone having designing details how these higher order bands can be tuned.....Reply Back
If your calculations are correct for winding the helix, you will have a very narrow band antenna that resonates at 59MHz. Without some complex or lossy matching it will present a very poor VSWR at all but the resonant frequency. On dual band VHF/UHF portable antennas its common to use a helix for the 150MHz band to shorten the antenna and then attach a full length 1/4 wavelength monopole for 450MHz that resides inside the VHF helix. If you can figure out how to match your 59MHz helix for 30-88MHz it would be plenty long enough to house separate but parallel 150 and 450MHz 1/4 wavelength monopoles within the larger helix.
The design of a multiple frequency helix is very complex and way out of my league. Without a conjugate matching network, your example of a 150MHz helix with a 450MHz whip inside will only be useful at two discreet frequencies with a small percent BW and 30-88MHz will not be a consideration. You may be better off with a long flexible whip for 30-88MHz tied to shorter whips or helices for the higher freqs. A matching network will be very complex to bring this all together or you might consider lossy resistive matching. Do you have a design goal for the maximum antenna dimensions? How far do you expect the radio and antenna to operate? You might also ask these questions on the news group rec.radio.amateur.antenna.