Topband: Coil position

[k3bu at optonline.net]

----- Original Message -----
From: Rick N6PE <n6pe at pacbell.net>
Date: Saturday, May 13, 2006 1:08 pm
Subject: Topband: Coil position

> Something someone said the other day about a top loading coil on a 
> vertical acting like a mobile antenna raises a question.
> 
> We have all seen the theory and computer program calculations 
> telling us top loading is better than base loading.  
> My question is, given a 32 foot support, has anyone run any on the 
> air tests comparing a base loaded 32' vertical with one that has 
> the coil 6-10 feet from the top end? 

Yes, there was a number of mobile antenna shootouts, when various configurations and ways of loading were tested and field strength measured. W5DXP had some tabulated results I believe on mobile reflector or antenna NG.  The most pronounced effect is on the low bands, 80 and 160 m, where those differences are "magnified" and efficiency is worse.  Using longer radiator improves the efficiency but the "loading rules" remain the same.

 
> The top coil has to be larger in uh (and probably loss) than say a 
> big fat base coil made out of 1/4" copper tubing. 
> 
> Is there any "real world" detectable difference in signal level ? 
> 
> I'd be real interested in hearing from anyone that has done any 
> actual on air tests. 
> I've run all the EZNEC charts and have seen all the articles in 
> QST etc.
> 
> > I suppose the next question is would it than be better to 
> eliminate the coil and use 4 top wires to load the vertical?
> 
> Thanks,
> Rick Darwicki  -= N6PE =-

The whole idea about loaded, shortened antennas boils down to the current distribution along the radiator. The efficiency of loaded antennas is proportional to the area under the current curve along the radiator and loading elements. The more, higher current we can supply over the length of the radiator, the higher efficiency, more signal we can radiate/receive. If you understand that, then all is clear (as a mud :-)

In the resonant radiator (as seen at the feedpoint) the reactance has to be zero, then you get sine wave distribution of the current along the radiator - maximum at the base, zero at the tip, with voltage distribution just opposite. This is due to the standing wave along the radiator (antenna IS the standing wave resonator). Now we come down to how do we affect the current distribution along the radiator and how to maximize it.

The best way is to have top loading - top hat, T or L loading. This provides the maximum current along the radiator - the "fat" portion of the sine current curve. With top hat or T loading the remaining portion of the current (horizontal pol. component) is cancelled out and in vertical radiator we end up with vertically polarized signal. With L loading we get some horizontal polarization component, proportional to the amount of current in wire from the bend on.

The next best way of loading is by inserting the coil along the radiator (can't "afford the hat"). To understand the behavior of coil in standing wave (antenna) environment we have to realize that depending on the insertion point along the radiator, coil will provide current drop along its length. It appears that impedance of radiator and impedance of the coil interplay. (More info on the "great loading coil controversy" is at http://www.k3bu.us/loadingcoils.htm)

The best position for the loading coil is closer to the top (about 2/3 to 3/4 up) of the radiator, this maintains "fat" current curve above the feedpoint, then we have some current drop along the coil, with appropriate remaining current drop towards the tip and to zero.  Again looking at the current distribution curve gives us visual picture of the antenna efficiency. The higher the coil the more turns required, matching the impedances (or bumps in them) at the insertion point and maintaining resonance.

Sliding the loading coil down, will show that the bottom "fat" portion of the current curve is shrinking, current along the coil drops and remaining "skinny" curve towards the tip has less area than when the coil was higher (rectangle vs. triangle). 

The worst position for the coil is at the bottom, base of the antenna, it "eats" the good portion of the "fat" curve, with almost triangular drop to zero, from the top of the coil to the tip of antenna. The difference between the areas under the current distribution curve and corresponding efficiency are quite obvious. There is some difference in number of turns required for base vs. mid loading coils and corresponding ohmic losses, but those are less significant than losses from the differences in the current distribution along the radiator. This is well known and confirmed by tests and proper modeling.

When modeling loading coils in EZNEC, by using just lumped inductor, EZNEC shows no current drop across it and distorts the picture. You need to use either loading stub of the same inductance, or EZNEC solenoid feature (some limitations) and then you would see the drop of current along the loading element and the remainder of the radiator.

The next thing for efficient loaded antenna is the matching of its low impedance to the feedline. In my mobile setup I use few turns of heavy copper tubing coil, just enough to get 50 ohms across it. This also provides DC path to ground for static or atmospheric charge. Other means like ferite core transformer, or LC networks are used. 

As to your 32 foot radiator, on 160m I used my 40m vertical, at the top I inserted loading coil and extended it with L loading wire, trying to get about 3/8 wave L loaded radiator. The base was elevated about 4 feet on the edge of the Barnegat Bay (brakkish) water and had one 1/8 wave elevated radial. I trimmed/tuned them them to close to 50 ohm feedpoint impedance and used balun.  This arrangement gave me the maximum obtainable "fat" current curve for the available vertical and vertical polarization signal and also some "free" horizontal component. Worked so well that I managed to beat US QRP records with this "crippled" antenna.  I am planning to use similar arrangement for the phased arrays at the Bay there.

So if you are land-locked, I would use similar setup, just lay as many radials underneath the antenna. If you are after just vertical polarization, I would use T or X loading on the top and two or four elevated radials, loaded if necessary, playing by the above "rules". The trick is to get the maximum of current curve along the radiator, minimize the losses, ohmic and ground. The higher and fatter the current curve - the more efficient antenna we get. Stick it in the salty mud and get 10 - 15 dB free. That's the "secret".

I hope this clears some RF mud.

73
Yuri Blanarovich, K3BU, VE3BMV