what's the science behind loading coils?

[LIScanner101]

I see that most of the venerable multi-band scanner antennas like the vintage A/S MON-Series and the contemporary Austin Spectra all have loading coils. The Spectra has two, the old MONs only had one big one.

How are these coils characterized? I understand that you can lengthen/shorten the metal rods to change the VHF low band and/or high band performance, but doesn't the coil have an impact on that? Is there a set of formulae that one can use to wind these coils to get the proper resonance/reactance/whatever? I'm not an antenna guy but I'm very interested in the theory both theoretical as well as practical.

 

[prcguy]

You have opened up a very interesting topic but first you must identify what the coil is doing before you can characterize it. In the A/S MON series I believe the coil is actually a coil and capacitor making up a parallel resonant circuit at around 155MHz to make the lower whip appear to be a 1/4 wave resonant on VHF hi band.

The parallel resonant circuit would be called a "trap" and it also acts as a loading coil for the upper whip to make it resonate somewhere in the VHF lo band without having to be 7 or 8ft tall. I do not know for sure this is how A/S designed the antenna but its what I believe from studying them.

On the Austin Spectra there is only one coil in the upper part of the antenna which is doing basically the same as what the coil (trap) on the A/S MON is doing but I believe the lower coil is not a coil but simply a length of dense plastic (Delrin ?) and is used as "dielectric loading" to simulate what a trap does but at higher frequencies (like 800MHz).

For a traditional loading coil which is used to simply shorten the length of an antenna element you can find many calculators online like this one: Antenna Loading Calculation

When designing a shortened loaded whip you usually need to know something about the ground or counterpoise that will react with the antenna, then run the calculator to get an idea of the coil size and shape, the best location along the whip, then bring up other calculators to find how many turns of wire is needed to achieve the needed inductance for a particular size coil.
prcguy

I see that most of the venerable multi-band scanner antennas like the vintage A/S MON-Series and the contemporary Austin Spectra all have loading coils. The Spectra has two, the old MONs only had one big one.

How are these coils characterized? I understand that you can lengthen/shorten the metal rods to change the VHF low band and/or high band performance, but doesn't the coil have an impact on that? Is there a set of formulae that one can use to wind these coils to get the proper resonance/reactance/whatever? I'm not an antenna guy but I'm very interested in the theory both theoretical as well as practical.

 

[LIScanner101]

VERY interesting stuff here. Thanks. However, it does appear a little intimidating for an average hobbyist. I had no idea this was such a complicated subject, but I guess in retrospect I should have anticipated that. Again, thanks for the good info!

 

[prcguy]

The more I learn about antennas the more I learn how much I don't know about them. They are far more complicated than most people think and unfortunately lots of bad and misleading info is being propagated by people with good intentions. Listen to some antenna conversations on CB and you'll get an earful of that.

If you can find a mentor to help understand some basics then learn how to drive the antenna analysis EZNEC and characterize some common antennas, you can really start to see whats happening.
prcguy

VERY interesting stuff here. Thanks. However, it does appear a little intimidating for an average hobbyist. I had no idea this was such a complicated subject, but I guess in retrospect I should have anticipated that. Again, thanks for the good info!