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QCX 40 ~ 1/2 wave NVIS Observations & question #antenna

Steve Raas - N2JDQ - FN13DD
 

Good morning everyone. This morning while debugging and repairing a problem with my QCX 40, it was time for an on air test. After running my coax to the antenna on the fence and checking the SWR , I started Cqing around 7017. SWR when I started was 1.4 or so...fine for me. Then the rain started,  not any downpour,  but enough to moisten things. This is when I noticed my SWR creeping up slowly.
I pondered this for a moment and decided to test a theory, and it appears that with the antenna wet  the resonant frequency goes down considerably. About 500 khz. 
I'm wondering if any one knows if this is a typical reaction with a low mounted halfwave? Or any half wave? Or any antenna regardless of its physical attributes? My gut feeling says, that due to its low proximity to ground, when ever ground conductivity changes significantly, resonance may or will change as well? Also, is there any tip or trick to combat this temporary issue? I'm thinking about folding the ends of the dipole elements over on themselves 12-15 inches to see if I have any immediate result. Unfortunately the elements are not bare copper...so again 'my gut' says this would not do a thing.

Steve Raas 
N2JDQ 

Richard G4TGJ
 

My aerial is a doublet thrown over the roof with the ends in trees. I have to retune it slightly when it's wet. So I'd say it's normal. Since I'm using an ATU anyway it's no big deal for me.
--
73
Richard
G4TGJ

Steve Raas - N2JDQ - FN13DD
 

Richard, 
Thanks for reading and replying so fast. I tend to agree, and I decided to test out my folding over the ends idea. 

Elements are 18awg insulated copper wire, and I'm unsure of the exact ammount that I twisted the ends upon themselves, it appears to have brought the resonant frequency back up. My guess is 14 inches per side. Hmm..i was thinking it would have no effect, but now that my SWR is back at my t ollerable level....back to cqing!

I'll have to remember this, as I'm sure it won't be the last time I have a wet antenna and no tuner. 

Steve Raas 
N2JDQ 

ajparent1/KB1GMX
 

Steve,

Question is this a real 67' half wave or is it a 1:9 balun fed long wire?  

As someone that was in antenna design before retirement...  Yes, ground surface conductivity
can vary greatly depending on soil, location and moisture level.   Of course over salt water 
to something like dry like desert sand will show a great change.  There are two parameters that
change one is the basic conductivity and the other is the dielectric loses.  

A half wave feed point impedance regardless of end fed or center fed change with height 
and ground conductivity under it.  There are charts in the handbooks showing this.

For the random wire form you have the effective length due to effective height and also
due to ground conductivity and grounding (either via radial or counterpoises).

Is there a fix?   You might try a counterpoise at ground level as long as the antenna.
Its not a "fix" but it may reduce the amount of change.  Not if the antenna height
changes from water loading that also will impact the result.  

FYI: NVIS at 7mhz is unlikely as the critical frequency especially at low sunspot part
of the cycle is well under 7mhz.  Heck the 75M NVIS coverage this AM was terrible
due to the current level of geomagnetic upset (aka spaceweather).  An NVIS
height is typically 1/10th wave high (about 12-14FT) and the feed point impedance
is often far from the normal for say 30-40 feet up.  What is being done is using
the ground as a reflector (or a wire) so the pattern peak gain is near vertical
and then you depend on the ionosphere to be reflective at the working frequency.

An endfed half wave as a sloper [~45 degrees) would do better as it has a small
amount of directivity and less prone to surface effects.  It does mean you need
to get the far end 50-60ft up.

If you looking for something easier to deploy try grass wire, no height at all.
Try this, K3MT grasswire.  His claims are valid and I have used it many times when 
getting a wire up was impractical or for bands like 160M.  However like the short
stubby on the 2m HT done expect high efficiency.

Allison

Steve Raas - N2JDQ - FN13DD
 

Alison, 
It's a classic centered 40m dipole,  with no Balun. It is hand tuned to 'dry resonance ' , and far shorter than 67 feet. My guess is its closer to 55', ide have to measure it. As far as deployment goes, it stays on the fence,  I just run the coax when I want to operate, which hasent been much after kidney surgery. Be that as it may be, I'll take a look at this PDF you sent. I think once the WX gets a little better, I will do more antenna experimenting,  as i want to try a full wave wide diamond loop with a 75 ohm matching stub.
Steve Raas 
N2JDQ 

ajparent1/KB1GMX
 

Steve,

Hope your recovering well.  Yes even the common center fed dipole will show the same effect.

Due to the fence and being close to the ground (at 40M anything under 10Ft is close to the ground)
the 55ft length sounds about right. On the ground that might be as short as 25-30ft due to loading
effects. 

The other part is the fence being plastic or wood when wet may be as much a factor as the dirt holding

Full wave loop, 143ft of wire.  Likely to work.  Loops are not as fussy close to the ground.

Allison



it up.

ajparent1/KB1GMX
 

Steve,

One last if you support the wire just off the fence (when you can) it may be less fuzssy in the wet.
I'm talking inches away, using say electric fence insulators.  I did that using trees and 5" Electric
fence insulators to keep the wire from actually touching the trees (75M NVIS ~112ft long).

Allison