Posted by
Don Wilhelm-4 on
Feb 04, 2010; 1:59pm
URL: http://elecraft.85.s1.nabble.com/OT-Interesting-Accident-with-Homebrew-Vertical-tp4511024p4513561.html
Steve,
Elevated radials are not the same as buried radials - while related,
they are not the same. Elevated radials must be resonant, but in-ground
radials are untuned - those simply make the earth a better ground screen.
If the desire is for the antenna to be resonant (so it is easily fed
with coax), one must tune it. We are normally using 1/4 wave radiator
and 1/4 wave radials, but they can vary slightly from that exact length
and cause no problem We want the whole thing to be resonant, and that
can happen with a slightly long vertical section and slightly shorter
radials (or vice-versa) - think about it as a dipole fed slightly off
center, the feedpoint impedance changes a bit, the the dipole can still
be resonant.
You can tune the vertical element, or you can tune the radials. I
prefer to tune the radials rather than bringing the vertical element
down for pruning. Yes, there will be ground effects, and the easiest
way to do it is to cut the radials long and trim as required - that
compensates for any effects of ground and surrounding objects.
AND, if you want equal currents on the radials, each one must be tuned
to the same frequency (same electrical length), so tune each one in turn
with the vertical element, then connect them together at the feedpoint
when that has been finished. See ON4UN's Low-Band DXing Chapter 9
section 2.2.7 for details on this technique (the theory and discussion
is in section 2.2.6).
As far as cancellation of the radiation from the radials, yes, that will
happen if they are oriented in pairs 180 degrees apart. I refer you to
the writings of LB Cebik and others for verification. It does happen in
theory and modeling - how close it is achieved in practice is another
question.
73,
Don W3FPR
Steve Ellington wrote:
> I think the original question was.....Can you feed the vertical section with
> the braid and radials with center conductor?
> I guess technically you could since neither one is grounded. It's weird to
> think about.
> Also I'm not too sure about all that "radial tuning" and canceling of
> horizontal radiation if the radial system is only a few feet from the dirt.
> It's interesting to think about how radiation is "canceled". Where's it go?
> Think I'll go to bed and dream about this. 73
> Steve
> N4LQ
> ----- Original Message -----
> From: "Don Wilhelm" <
[hidden email]>
> To: "Robert Fish" <
[hidden email]>
> Cc: <
[hidden email]>
> Sent: Wednesday, February 03, 2010 11:06 PM
> Subject: Re: [Elecraft] OT - Interesting Accident with Homebrew Vertical
>
>
>
>> Bob,
>>
>> Indeed elevated radials *are* a part of the antenna (actually radials in
>> the ground are too). Elevated radials must be tuned (while buried
>> radials do not need to be). To do that properly, connect each one - one
>> at a time - and resonate it with the vertical element. When all have
>> been tuned, then they can be connected together.
>> To cancel the horizontal radiation component, the radials should be
>> oriented in opposing directions - any pair should be in a straight
>> line. 4 radials arranged 90 degrees apart is normally sufficient, and
>> in a pinch, 2 placed 180 degrees apart are sufficient.
>>
>> It matters not whether the vertical part is fed from the center
>> conductor or the shield. The currents on the inside of the coax are
>> balanced (equal and opposite currents). The "magic" is to keep the
>> current off the outside of the coax shield, and that is what common mode
>> chokes (current baluns) should accomplish.
>>
>> 73,
>> Don W3FPR
>>
>> Robert Fish wrote:
>>
>>> I noticed
>>> that I had hooked up the shield of the coax to the vertical element and
>>> the center conductor to the radials. (I was experimenting with running
>>> 180 deg out of phase for end fire and forgot to change it back). I am
>>> not sure what that proves except that he radials really are half the
>>> antenna. Working Africa from here on the west coast with only 100 watts
>>> is pretty rare these days, so the backwards hookup is obviously working.
>>>
>>> I guess it makes sense that it shouldn't matter. What do you guys think?
>>>
>>>
>>>
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