[Lowfer] Re:helically loaded verticals

[Tom Rauch]

> voltage. Elevating the loading coil will move the high voltage parts
> of the antenna away from the ground, reducing dielectric losses in the
> ground and things near it, and so reducing Rloss. For a given antenna
> and top hat, the effect of dielectric loss will be greater at LF than
> at HF because the reactance of the antenna, and hence the voltage for
> a given antenna current, increases as the frequency is reduced. Rloss
> appears to be dominated by dielectric effects at LF, due to the high
> impedance nature of LF antennas. Measurements on several amateur LF
> antennas has shown that Rloss is very roughly inversely proportional
> to f, at least over the range where the antenna is small compared to a
> wavelength. So what happens at 1600m is not quite the same as on 160m,
> because of the difference in scale, so having helical loading, or an
> elevated loading coil could be an advantage on LF.

I respectfully disagree. Nothing changes. It all works the same 
when everything is scaled to the same dimensions as a fraction of 
wavelength. That is why formulas do not change from VLF to VHF 
and higher, and why modelling programs can use the same engine 
no matter what frequency we are on.

My 8-foot 160-meter mobile antenna has exactly the same 
problems as an 80-foot 1600-meter antenna, and the problems can 
be handled the same way electrically. Only the mechanical 
aspects, such as physical size, vary. to have the same ground 
system as my F-250 truck, I would have to have a "land-barge" 
under the antenna. 

If I place the loading coil very low in a system with a large hat, 
power factor correction occurs in the coil. Voltage just above the 
coil is very high even through current (which is the mechanism 
behind desired radiation) is uniform throughout the system. This 
causes needless circulating displacement currents through the 
electric field (which we all know does not radiate any useful 
distance because it is an induction field), which has the effect of 
compressing base impedance of the system and increasing loss in 
both the inductor and any lossy media around the antenna.

If you have a large flat ground screen near the base of the antenna, 
these effects are minimized.  Once the inductor is a few feet (it 
would be more exact to talk in terms of very small fractions of a 
wavelength) above the groundplane, it can be anywhere in the 
system. The only two "sore spots" are right under the hat, and right 
at the feedpoint.

If you think about it, this is why we should never build a mobile 
antenna with the loading coil below substantial nearby sheetmetal. 
The sheetmetal "short-circuits" the system by providing extra 
coupling directly to "ground". like adding a shunt capacitor. Same 
for placing the coil under the hat. The hat will have capacitance 
back to the mast below the coil, and if we cause a large potential 
difference across that point we decrease bandwidth and increase 
losses. We, in effect, increase circulating currents in the coil 
without changing the rest of the system.

I really don't see any difference between the systems.      

> Most of us have found over here that the losses in a reasonably
> constructed loading coil will normally be swamped by other losses in
> the antenna.

Same on 160-meters. My truck looks like about 25-ohms. It is 
MUCH more important to get current distribution correct than to 
change inductor Q once I reach a certain value of ESR in the 
inductor.

As a matter of fact, the FS change going from Q=250 (like a small 
#14 airdux inductor) to Q=800 (about the practical limit using 
transmitting-type edge-wound inductors or copper tubing) is barely 
detectable. The change from having uniform current to triangular 
current distribution measures about 5dB!

I chuckle when I see super huge coils on 160 meter or 80 meter 
mobiles, because ground losses are so high the extra Q is a 
waste. Especially when they stick the hat right above the coil, and 
have a whip with hardly any current waving in the air above the hat. 

Bottom line is use a large hat, and don't shunt the inductor with 
needless capacitance. It works that way on any frequency.
73, Tom W8JI
W8JI@contesting.com