Hi Ash,
I see that you are evaluating your Beverage antenna by using
SWR. Have you actually adjusted the termination resistance to null
medium-wave broadcasters located off the back end of the
antenna? If so, what was the value of that termination resistance?
If not, it might be worthwhile to make that adjustment in order to
understand better your antenna. As others have pointed out however,
the Beverage will work well without optimizing its termination, but
it sounds as if you would like to understand its operation more clearly.
If your ground conductivity is very poor, much of your termination
resistance will be made up of the resistance between the ground
rod(s) and the earth, and that resistance can be substantial when
using short ground rods in poorly conductive ground. Perhaps only a
small value of termination resistor will actually be needed, even
though the impedance of the antenna is around 500 ohms. And, that
termination resistance will possibly vary with the season of the year.
I have had to use zero ohms termination at one site, as that provided
the best null for a medium-wave broadcast signal off the back end of
the Beverage antenna. Perhaps the resistance to earth at that site
was even in excess of 500 ohms, so I would have needed to create a
better system of ground rods to improve the null further. I imagine
the SWR of that antenna varied quite a lot with frequency.
The Saudi and Egyptian broadcasters that you will use for nulling are
quite powerful I believe, so I hope that they do not suffer from
short term fading at night that can make a null difficult to find.
best of luck with this project,
Nick
VE7DXR
At 14:33 2018-07-06, Ashraf Chaabane wrote:
Hi Mark, Peter,
The BC station to be used to minimize the back lobe would be in
Egypt or Saoudi Arabia right on the back of my beverage antenna.
That should offer signals coming well above 10deg (I presume!).
Comment noted regarding how efficient that will be on 1820 kHz. I
know Egypt has BC station on Medium Wave which isn't that far from 160m.
Frank,
I have two 3-feets rods at each end so far with two buried radials
at the feed end. I will add radials at the termination side as well
as you recommend.
For the sake of experimentation, I will play a bit with the
termination resistor trying to flatten the SWR curve as much as
possible, although, as many said, I shouldn't expect a noticeable
improvement in reception.
Thank you all and those who wrote me in private. This generous
technical support is needed for developing experience on 160 among
3V hams (mostly youngsters). I hope we will be ready with strong
100W signals this winter!
73 from Tunisia
Ash 3V8SS/KF5EYY
www.kf5eyy.info
------ Original Message ------
From: donovanf@starpower.net
To: topband@contesting.com
Cc: "Ashraf Chaabane" <ash.kf5eyy@gmail.com>
Sent: 04/07/2018 19:00:33
Subject: Re: Topband: Beverage Antenna Tuning
Hello Ash,
Your results are normal for a typical Beverage. Receiving performance
is not significantly degraded by imperfect matching of the coaxial feedline
to the Beverage feed point or by non-optimum termination resistance.
You could optimize your matching transformer and termination resistor
but the improvement in receiving performance will be very minor.
On the other hand, Beverage antenna receiving performance can be
significantly degraded by:
- nearby antennas, power lines and other long conductors, and
- common mode signals coupled into the feedline
You could optimize your matching transformer to reduce the VSWR to
closer to 1:1 on the frequencies you care about most, but you won't
notice any improvement in receiving performance.
You could optimize your Beverage termination by:
- adding extra ground rods and/or short radials to improve
ground resistance
stability at the connection to termination resistor.
- adjusting the termination resistance to minimize the VSWR excursion
as you sweep from 1.5 to about 10 MHz
73
Frank
W3LPL
--------------------------------------------------------------------------------
From: "Ashraf Chaabane" <ash.kf5eyy@gmail.com>
To: topband@contesting.com
Sent: Wednesday, July 4, 2018 2:13:27 PM
Subject: Topband: Beverage Antenna Tuning
Hi All
I measured the SWR in my beverage antenna; the SWR fluctuates between 1.5
and 3 in a range of 1.8 to 7 MHz. (See:
https://drive.google.com/drive/folders/0B9AtNpPfAOUMRDNsaXROMG1VUTg?usp=sharing
)
I know SWR should not vary too much. However, some people are suggesting
adjusting the termination resistor. Others suggest checking the transformer
number of windings for a good match. What shall I do?
73 Ash 3V8SS/KF5EYY
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Nick Hall-Patch
Victoria, BC
Canada
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