> From: Frank Donovan <donovanf@sgate.com>
To: <towertalk@contesting.com>
> Date: Tue, 21 Oct 97 11:27:19 +0000
Hi Frank,
> No, I have not personally checked Beverage sidelobe response with and
> without sloped terminations;
I have in Sylvania, Ohio.
I had two reasons for doing this. A high tension line about one mile
from my NE and NW Beverage. I was also trying to maximize F/S ratio
so I could work JA for WAC through the loud Lorain signals on 1900
KHz.
I found absolutely no difference on receive (receive tests are
hard to make, and very subjective), or on transmit when the
Beverages were excited with a low power transmitter and FS readings
were taken (these tests are easy to make and very accurate).
The response to vertically polarized signals was exactly the same,
and the pattern had all the same peaks and nulls on groundwave.
> This receive Rhombic array always had problems with clutter from radar
> return signals entering thru the many side lobes in the stacked Rhombics.
> Extensive research was conducted in the 1970s and after extensive
> theroretical and experimental work a new array was constructed to replace
> the big receive Rhombics, with the peak sidelobe at -40 dB relative to the
> main lobe.
I can imagine so, I wonder why they even tried that if they thought
through the problem. (The OTHR system I designed PA's and exciters
for (around 1980) used short terminated broadband elements and
measured the phase shift between them, it was not one array).
Rhombics have terrible sidelobe pattern and very poor efficiency.
The only measured data I have seen is for a V beam (from General
Dynamics data), but the principles are the same......the Rhombic is
just two end-to-end V beams. In one example the sin X / X comparison
of a 3 WL long V shows it is about 8 dB weaker than the signal from
the same main lobe HPBW should produce. That 8 dB comes from
some ohmic losses, but mostly from the horrid side lobes.
I can't imagine why any engineer that really understands antennas
would ever use a nasty side lobe antenna for RADAR! They must have
used 50 year old textbooks!
An array of broadside elements would cancel the side lobes
while adding the main lobe, sloped ends or not. Maybe that was the
real solution.
> I accepted the work of the engineering team that designed that Beverage as
> the basis for my own, and mine work extremely well!
Perhaps looking at this using what we DO know about the Beverage
would help. If an antenna is 500 ft long, has eight foot high ends,
and carries uniform current exactly how much extra signal
contribution would we expect from eight foot of wire out of a
total of 516 feet? No much in my estimation!
In my measurements here and in Ohio, the far end current of the
Beverage was between 50 and 75% of the feedpoint current, so the
termination end drop wire would do even less. It is coupled to the
feedpoint end through what amounts to a 3 to 6 dB attenuator, the
antenna itself.
Finally what difference in sensitivity results from spreading an
eight foot drop over a small distance, verses just dropping it
eight feet at one point when both phase shift and current taper are
very small in that antenna area? Either way, we still have eight feet
of vertical drop. Either way, the current is almost exactly the same
and very little phase shift occurs on 160 in less than 100 feet.
>From everything I've learned in using and measuring Beverages and
other receiving arrays since the mid-60's, the sloped ends probably
have as much advantage as the "shielded drop wires" some books
INCORRECTLY claim help "reduce vertical end pick-up".
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|