What is a loading coil?

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poltergeisty

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I have this antenna and it performs better in HF than this longer one I bought on eBay for $20! I think it has something to do with the loading coil but I don't know what that is. Can anyone describe what a loading coil is?

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W8RMH

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The term is used for inductors in antennas, or between the antenna and its feedline, to make an electrically short antenna resonant at its operating frequency.

A radio antenna, usually shorter than a quarter wavelength for practical reasons, presents capacitive reactance to a transmission line. This can be canceled by inserting an equal and opposite inductive reactance in series, by means of a loading coil typically at the base or center of the antenna. Consequently the antenna presents only a resistance to the transmission line; this is desirable because it minimizes the energy reflected back into the line.

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prcguy

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To further what W8RMH said, besides allowing for an antenna to be shortened and still achieve resonance, some loading coils provide different functions like acting as a choke or trap at a certain point of an antenna so part of the antenna will resonate at one frequency and the entire length will resonate at another freq.

In the case of VHF/UHF antennas a loading coil or coils placed along the length of the antenna can actually be phasing lines to break up 1/4 wave, 1/2 wave/ 5/8 wave, etc stacked elements and provide a 90 deg phase shift between resonant elements so they all work in phase to provide gain.

The antenna provided in the first link looks similar to an old Radio Shack scanner whip and the coil loaded the antenna to resonate on VHF lo band. You might measure the distance from that antennas connector to the base of the coil and if its in the 16 to 18in range the coil may also break up the whip so the lower section acts like a separate VHF hi band antenna. If this is true that lower section would also work (somewhat) as a 3/4 wave UHF antenna, so with the cleaver placement of a coil the antenna could possibly pass as a tri band antenna.
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kayn1n32008

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So since the antenna uses a loading coil to make it resonate with a shorter antenna how does this apply to receive only?

While resonance for receive only is not as critical, as it is for a transmiter. To demo why receive resonance is still important try this little experiment:

Find a UHF signal that is rather weak even with a properly tuned UHF gain antenna(5/8 over 5/8 wave ect)... Then switch that antenna for, say, a VHF 5/8wave or a VHF 1/2wave (not a VHF 1/4wave) an antenna that is not resonant on UHF and listen to the same weak UHF signal.




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kayn1n32008

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I don't have those antennas. What is the outcome? I do have a few radios that are UHF, but the squelch is programed and the connector is SMA.

Simply to demonstrate how antenna resonance affects receive signals. Outcome would most likely be that the UHF signal would not be heard with the VHF. Antenna on receive.


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poltergeisty

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What I find odd is that I have several scanner antennas for my handhelds and I have been having trouble getting the towers. The best antenna that works so far is an old VHF antenna that was from a VHF two way green dot radio and it performs quit well on 800 MHz. I don't understand that. This antenna is a rubber duck type which is coiled. I'll have to take a pic latter on so you know what I'm talking about.
 

poltergeisty

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Got some pics.

This is the VHF antenna that performs quit well on 800 MHz.





This antenna doesn't perform as well on 800 MHz. Is that a loading coil on the bottom?




This antenna came from a WIFI adapter and I bought a SMA to BNC adapter for it thinking it would perform quit well since the antenna was for high frequency.

 
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