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Old 05-12-2017, 10:15 AM
me1900's Avatar
Join Date: Jan 2017
Posts: 107
Wink Using Harmonics of CB Antenna for Reception

I heard about every frequency has a harmonic of itself multiplied by a number. I do not know the equation but being adventurous I figured a cheap cb antenna might serve my purpose on certain frequencies. Listen to me please if you have a critical thoughts against this thread I am only trying to bring forth an alternative for those who might not have means but do have a cb antenna laying around doing nothing..

I am experimenting with a $15 Walmart antenna.

Since CB antennas are tuned for the 27mhz range.I am trying my luck on these harmonics of 27mhz with my experimental antenna.

27mhz frequency input (CB band) BEST RESULTS

2nd harmonic 54mhz (Top of 6 meters) NO RESULTS NO CONDITIONS

3rd harmonic 88mhz (bottom edge of FM Broadcast) GOOD RESULTS

4th harmonic 108mhz (Top end of FM broadcast. Start of Aviation band) GOOD RESULTS

5th harmonic 135mhz (Top of Aviation band) FAIR RESULTS

6th harmonic 162mhz (NOAA Weather Radio area of the dial) GOOD RESULTS up to 50 miles

7th harmonic 189mhz NOT TESTED

8th harmonic 216mhz NOT TESTED

10th harmonic 270mhz (Mil Satcom but not a good idea to use cb antenna for this) NOT TESTED

I do not think this would work using a high quality CB antenna. It may work on a low end cheapo like mine
Its worth a shot if you do have a cb antenna laying around to get ya started scanning CB to VHF.

Post your results.

Edit: May I add that I am using RTL-SDR NESDRSmart and HDSDR for the receiver in this test. Your mileage WILL vary!

Last edited by me1900; 05-12-2017 at 10:21 AM..
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Old 07-14-2017, 7:55 PM
Join Date: May 2009
Posts: 2,015

Keep in mind that the impedances and radiation/reception patterns change for the harmonics. Basically, each harmonic "sees" a different antenna.

Example. At #1, the fundamental, lets say it is your typical 30-50 ohm impedance quarter wave.

At #2, the 2nd harmonic, what *it* sees as an antenna is now a high-impedance, poorly matched end-fed half wave! No wonder it doesn't work.

As you move up in frequency, what the harmonics see start to approach "long wire" antennas, even if they luck out with a low impedance match.

The problem with long-wires, is that unlike a quarter wave or half-wave, the radiation / reception patterns starts to have many thin lobes and nulls, and typically pointed nearly straight up and down!

If you are very very close to the transmitting source, this may not be an issue. With cheap scanners with wide-open and poor front-end circuitry, this degradation of true antenna performance may actually seem to help, so be careful about proclaiming victory.

In other words, the antenna working on harmonics may be very poor, but under certain conditions, like a poor receiver front end and very close proximity, from a *system* standpoint it seems to work.
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