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Author Topic: Very very odd issue with Antenna Installation - Tarheel as a Base  (Read 2447 times)
KD2CJJ
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Posts: 369




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« on: May 01, 2013, 10:09:21 AM »

Hi,

I have a family member who is experimenting with his Tarheel screw driver antenna as a base antenna in a HOA.   He has cut a hole in his roof, put the antenna in a PVC tube to extend the antenna beyond the roof.  This is a slanted roof and the antenna is in the middle of the roof between the peak and the base.  His roof is tile barrel roof, with ply under and lined with a radiant barrier under that.  He has attached the counterpoise/shield lug to the radiant barrier as counterpoise instead of using tuned radials (as I suggested).  His station is grounded to two ground rods out the shack which are connected to MFJ lighting arrestors which the coax goes through.  He has achieved a great ground at 5 Ohms (yes very good ground).  

The antenna tunes to a good SWR  down to 40m.  20m being his band of choice is roughly 1:2 SWR .  After doing many tests we can clearly see that the antenna has "no ears".  He hears virtually nothing across all the tuned bands...  BUT his transmit seems relatively OK.  As an example we tested the antenna transmitting to a buddy of his a few miles away.  He received a report of a 59 using the telephone... BUT he could not hear the station over the air.  The transmitter on the other side was using a 1/4 wave vertical and even tried over his full size 3 element beam up 50 feet --- he heard nothing!....   We have tried many radios and all exhibit the same issue thus its not the radio.

OK.... So the question is, why would his transmit be stronger then his receive?  Any ideas?  What could completely wipe out the receive but have little negative effect on the transmit?  I am puzzled.  Could the fact he is not using tuned radials be causing the issue?  and if so why not affect the feed point impedance OR transmit signal?
« Last Edit: May 01, 2013, 10:35:23 AM by KD2CJJ » Logged

73

Mike
KD2CJJ
N6AJR
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Posts: 9927




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« Reply #1 on: May 01, 2013, 10:44:13 AM »

Something is not right.  He should be able to hear something there. Try a dipole on 10 m hung up in the shack to see what is going on.  a pair of wires , about 8 feet 4 inch on each side, with on connected to the center of a piece of coax and the other piece hooked on the shield, should tx on 10 m and hear something. 
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W5WSS
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Posts: 1783




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« Reply #2 on: May 01, 2013, 10:51:36 AM »

There is a reason but one must determine the antenna pattern developed relative to the effects from the building and frequency applied to the antenna.

I suggest a fellow ham in a mobile drive around encircling the area about 5 miles out and transmit a consistant signa. Your friend can then begin to see what directions he hears incoming direct signals.

He may notice some issues with point to point communications.

Skywave signals differ from point to point and he may find that he hears skywave arriving from relatively higher angles downwards onto his vertical.

Point to point and low height relative to both antennas being used suffer some black out spots could be topography,buildings , clutter usually adding up to the sum total of absorbtion attenuation.
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KD2CJJ
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« Reply #3 on: May 01, 2013, 12:01:07 PM »

We did (end fed) and it heard 100% better... When A/B switching its like the Tarheel isnt even connected to the feedline.   Its not just a NVIS or line of site issue.. it literally doesn't hear anything!  Even Far away like another state or even countries.. it literally sounds like a dummy load, until he transmits!  The transmitting is getting out... He just cant hear the reply.

This is a very odd situation which makes no sense...

That's a good idea about driving around to see if maybe the radiant barrier is attenuating all the receive signals in one direction... but if that was the case then why wouldn't it affect his transmission?
 



Something is not right.  He should be able to hear something there. Try a dipole on 10 m hung up in the shack to see what is going on.  a pair of wires , about 8 feet 4 inch on each side, with on connected to the center of a piece of coax and the other piece hooked on the shield, should tx on 10 m and hear something. 
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73

Mike
KD2CJJ
W5WSS
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Posts: 1783




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« Reply #4 on: May 01, 2013, 12:20:31 PM »

Perhaps mutual coupling is cancelling the receive signal prior to being induced into the antenna for reception.

Via reflection, refraction and diffraction.

But not affecting the near field transmission the same.

Buildings of conductors and such can cause non reciprocal performance between signals transmitted and received.

Ultimately one needs to experiment empirically then decide if the adventure is worth the return.
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KD2CJJ
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« Reply #5 on: May 01, 2013, 12:50:10 PM »

Here are the pics of the installation to give you a better idea...
https://skydrive.live.com/?cid=4b1a43a5cb046cca&id=4B1A43A5CB046CCA%21400&Bsrc=Photomail&Bpub=SDX.Photos&sff=1&authkey=!AB_EO1MY2WY0WnU
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73

Mike
KD2CJJ
WB0KSL
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Posts: 94




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« Reply #6 on: May 01, 2013, 03:34:32 PM »

From the photos, it appears that a significant portion of the antenna is underneath the radiant (aluminum foil?) shield.  That seems equivalent to enclosing a significant portion of the antenna inside what amounts to a faraday shield.  The base of the Tarheel needs to be above the radiant shield.  Hard to say for sure, but that's what I'm seeing from the pics.

73 de wb0ksl
John

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KG4RUL
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« Reply #7 on: May 02, 2013, 04:53:16 AM »

Absolutely, the portion of the antenna beyond the point where the finger-stock contacts coil must be above the tile and flashing.  And, what is the material of the tube enclosing the upper portion of the antenna?  Is it painted with a metallic paint?
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W5WSS
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« Reply #8 on: May 02, 2013, 06:36:55 AM »

Antenna system performance:

Hello yeah, The base of the Tarheel or any antenna like this should be above the foil.

use a schedule 40 or better non conductive pipe to protrude just above the tiles.

use the pipe and a weather head as a chase for the control line and coaxial cable .

Attach the Tarheel to the non conductive pipe.

The antenna is now able to radiate from above the inverted plane if you want to use it.

But one can run tuned pairs of radials that slope downwards in opposite directions routed symmetrically for example.

They (The Pairs per band) can be separated by 6" and still work to return the power to the Tarheel and be an effective radial system.

Otherwise The existing foil "as is" though not a flat plane, can be utilized should one need to avoid visible wires on top of the roof.

For lightning and safety:

For safe entry relative to lightning and safety the entrance of the antenna coax and Tarheel control line is through the roof what provision is he taking to route a ground wire from just before the roof entry point?

Can he attach a lightning arrestor at the Tarheel feed point

and

From before entering the inside of the roof but lying atop one of the tile wells Can he run a
large ground wire to a ground rod?

For the station ground can he run a wire from the entrance panel or bus bonding bar located near the station to a nearby ground rod he may need to install?

And finally can he bond all the rods together including the ac mains rod making the rod to rod bond the lowest and least resistance pathway for lightning throughthe system to the earth ground?

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W5WSS
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« Reply #9 on: May 02, 2013, 07:03:44 AM »

For antenna performance:
 I left out one  question Can he relocate the antenna to the very near peak of the roof?

If not maybe he can elevate the antenna from there upwards to clearing the roof

Of course he will need to use a stronger pipe/w weather head like perhaps the diameter that best fits the existing hole and really brace it underneath using the roof truss and if possible some black colored guys made of rope.

Allot to rethink and if he pulls all this off he should have a decent system. Smiley
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AC2EU
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« Reply #10 on: May 02, 2013, 07:45:01 AM »

Is the shunt inductance high enough? Try removing it and test the receive. You should hear some signals with almost any kind of antenna on a receiver.. Do you have access to an antenna analyzer? something is very wrong with this "picture".
It may also be that the PVC and/or the paint is the problem. Putting an antenna inside of a dielectric changes things...
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KD2CJJ
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Posts: 369




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« Reply #11 on: May 02, 2013, 10:58:59 AM »

The challenge he is facing is that he lives in a HOA and is trying to make it look like a vent... it already protrudes above the roof line to some extent thus he can not raise it anymore without cutting the wip, which he will then lose some of the lower bands...

To give you guys an update, he spent some time on the phone with Tarheel and he removed the shielding around the coil that moves up and down thinking it was not necessary, allowing him to fit it in a thinner PVC tube.  Tarheel explained that it in fact is needed and in fact it would affect receive.....   With that said right off the bat he needs to fix this before we can say anything else is a contributing factor...   This will require a fair amount of work.... Maybe today or tomorrow we will get to test it with this shield on... I will report back and let you guys know how it works out...



For antenna performance:
 I left out one  question Can he relocate the antenna to the very near peak of the roof?

If not maybe he can elevate the antenna from there upwards to clearing the roof

Of course he will need to use a stronger pipe/w weather head like perhaps the diameter that best fits the existing hole and really brace it underneath using the roof truss and if possible some black colored guys made of rope.

Allot to rethink and if he pulls all this off he should have a decent system. Smiley
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73

Mike
KD2CJJ
KD2CJJ
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Posts: 369




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« Reply #12 on: May 07, 2013, 02:43:27 PM »

Wanted to give the thread an update.  We found the issue and it was a very good example why antenna analyzers are important, and why its even more important to know how to use one.

Unfortunately I am not installing this antenna and I have been assisting my dad over the phone and he is relying on another ham to do all the work and steer him in the right direction.  To make a long story short (and we are talking weeks now with with this issue) they have been purely focusing on Impediance as a factor of whether or not the antenna is efficient and resonant.  Yes their match was 50 Ohms and the SWRs were always below 1.5:1.  What they never took into account was reactance.  When I walked my dad through using the antenna analyzer over the phone even though his Z was around 50 ohms his X was not even close to 0, it was very inductive.  I can only speculate that where they tuned the screw driver antenna to, was set to a harmonic.  I had him raise the antenna until X went to 0 and guess what the antenna came alive! Noise levels increased significantly and signals were relatively booming in.  From there he was able to tweak the tuning to get a low SWR within an acceptable range of reactance - again 1.5:1.   Essentially if you know a screw driver antenna,  he almost had to DOUBLE the amount of turns before the antenna to get the antenna resonant again but with proper receive performance.

That plastic casing I described in my last posting is still being fixed - but it does NOT affect the receive.  It IS used though for the screw driver controller to determine the proper number of turns which is used to determine the spot at which you were last tuned for a specific band.  It continually goes off with out this on. 

Anyway... Lesson learned!
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73

Mike
KD2CJJ
WB6BYU
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Posts: 13578




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« Reply #13 on: May 07, 2013, 03:09:04 PM »

Are you measuring with the SWR analyzer at the end of the coax in the shack?  If so,
then the magic "X=0" won't help at all, because the impedance is transformed by the
length of the coax into some other impedance (but with the same SWR.)

If the SWR is low, R has to be close to 50 ohms and X has to be close to zero, because
any other condition will cause a high SWR.  Of course "low" and "close" are relative terms.

What probably happened in this case is that the coupling to the surrounding metal caused
a very lossy condition (though not necessarily a harmonic) at a particular setting.  Just as
with a tuner, you can often find settings where you can get a good SWR due to parasitic
resonances but much of your power is being dissipated as heat.


Glad you got it resolved!
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W5WSS
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« Reply #14 on: May 10, 2013, 06:38:11 AM »

Hello glad to hear that you fellas are working it out.

I looked at the photos and see that the antenna is well below the roof peak...And I understand the need to be as stealthy as possible, just know that many suggestions prescribed here are viable options especially raising the base height of the antenna to just above the peak and can be done from mostly inside the attic grrr hot etc and allot of work.

Nice job and have fun
73
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