Topband: 160 meter elevated vertical

[Tom W8JI]

Guy,

> I see K4XS as still being in the general case.  That is because a
> wavelength on 160 is 539 feet.  He wound up going up 55 feet (a separate
> TopBand thread) or 0.10 (a tenth) of a wavelength. That's short,
> wavelength-wise, even if we throw in the reasonable if arbitrary 1/8 wave
> point as the onset of growing "significant" issues with solenoids that 
> have
> gone completely nuts by 1/4 wavelength.

The reason I suggested a ground rod at the earth is to establish a ground 
point on the cable. No one has any idea how long the cable is before it 
reaches ground, but it is highly unlikely to have one right at ground.

If a system has 50 feet of effective cable length between the radials and 
the cable shield earthing ground and a velocity factor of light speed, it 
already has almost +J400 ohms. If the CM velocity factor is .7, or the 
earthing point is just 50 further feet away along ground, the CM Z is likely 
thousands of ohms.

The single most important thing we can do when suppressing CM on the cable 
into the house, and current into the ground, is establish a ground point on 
the shield so we are not "guessing" at what the system looks like.


>> A reactance caused by spooling cable into a "choke" might not be nearly 
>> as
>> effective, and can actually hurt the system, if the lead is long.
>>
>
> I completely agree, IF the lead is "long".  So far we have said 1/4 wave 
> is
> long gone nuts, and that you are starting to smell it at 1/8 wave.  But I
> don't want to be seen approving coax solenoids on 160 at all.

Air solenoids have a place, but only after we know what the system really 
looks like. I use they on my HF beams because on the down-lead side of the 
choke, I ground the shield to the boom. This avoids making the system 
accidentally **worse**, and still let's me be a cheap Ham using a coil of 
coax. :-)

If I wanted to stop CM up to the house or other gear, I sure would not plan 
on doing it with a choke alone.

> A 1/4 wave to ground on the cable forms a very effective isolation, and
>> even 1/8th wave is not bad, so in this case (or any other case without a
>> cable shield ground close to the antenna) we have to be careful.
>>
>> This applies to verticals and dipoles EQUALLY, because the interaction we
>> worry about is identical. There is no distinction between the two.
>
>
> radials? To me personally at least, the differences seem rather in the
> extreme. I could go on with the list.

It may seem that way to you, but it isn't. The choke functionality is the 
same with dipoles or verticals. The choke in every case functions to add CM 
impedance to the voltage source driving the CM current. It doesn't matter 
what that source is, the choke works the same.


> Agree, agree, agree.  And for K4XS' 55 feet, 1/10 wavelength is less than
> that 67 foot inception point at 1/8 wave.

This is where we get problems in real life. We make black and white "always 
do this way" for gray systems. We need to look at the system.

With just a fair ground at the bottom of 50 feet of hanging cable, a system 
already has a pretty high CM cable shield impedance at the radials. Without 
that ground, it becomes guesswork. Things transcend gradually in the world 
of standing waves, and are not at cutoff points where at 50 feet we do one 
thing, and at 70 feet we take another approach.

If standing waves were square waves, hard cut off distances would have some 
merit.

This is the same for dipoles or verticals, or even in receiving systems. If 
we have a long unpredictable lead to earth we better be careful what we do, 
because things might not behave like we think. When we have enough room for 
standing waves to stand, we better think about how they are standing.

I wouldn't throw an air-wound coil or a good ferrite core choke at anything 
I wasn't sure of, and that would be any system with a unknown significant 
distance to (or between) shield grounds. Establishing a ground point is an 
almost universally ignored golden first step on longer cables.

If we look at a popular balun book, to explain why a test dipole at 1/4 wave 
height did not benefit from a balun, the author cooked up a very strange 
theory about cable diameter in wavelengths relating to balance. The author 
neglected looking at the distance to the ground point of the shield. Not 
looking at the system trips a lot of people up.

73 Tom