On 12/3/2010 11:04 PM, Rick - NJ0IP / DJ0IP wrote:
OK Stuart. I guess what I have gleaned from yours and Jerry's post is, in
the case of a Beverage or similar RX antenna, it is both a DC and RF ground.
I have always been leery of running an antenna which needed a terminating
resistor for transmitting, but I have used Beverages for nearly half my ham
life (almost a quarter century) and I know that they work very well.
The last question that remains to be answered is, when you consider the
merits of a Rhombic antenna, even though it is terminated and you might lose
half your radiated power, the forward gain and low angle of radiation still
make it a very useful antenna.
Part of that computation is flawed because the simple computations used
neglect the reduction in current from radiation along the wire. The
longer the rhombic the less current reaches the end. K0EMS had a stack
of four extremely long rhombics for 2m EME, something like 680 feet per
leg and he found it was always unidirectional whether terminated or not.
Then there's the famous LaPort Rhombic that has a pair of assymetrical
rhombics with 5 and 7 wavelength legs. LaPort was an antenna expert at
Bell Labs. He designed the original for few side lobes based on simple
math in the 1950s and built it for overseas HF phone traffic for 5 MHz
(IIRC) and checked the pattern by flying an airplane and based the gain
claim of 27 dBd on the width and height of that main lobe. It worked for
AT&T. Its been in many VHF handbooks ever since. About ten years ago at
a Central States VHF conference there was a 1296 version tested at an
antenna gain measurement or contest. Came in at 17 dBd. The next year I
published an article in the CSVHF prodeedings titled, "Where's the other
ten dB?" With modern antenna analysis software like NEC based EZNEC, it
turned out that while the near side lobes in the horizontal and vertical
planes were reduced, there were huge sidelobes (in free space) in the
pattern at 45 degrees from the horizontal and vertical planes less than
10 dB down. I had the EZNEC make a file of signal powers over the whole
sphere and I wrote software to total them up outside the main lobe. I
found 90% of the power fed was radiated in the myriad of side and back
lobes including those four diagonal lobes. Which stands to reason when
you look at the full radiation pattern from a long wire that has
concentric cones in free space. You can only line up an array of such
wires to emphasis or cancel the first lobes, but there can be a couple
per wavelength of wire length. So I'm not a fan of a huge rhombic.
I don't think I would mind a termination resistor in that case.
Where I have a problem is with antennas like the T2FD antenna.
73
Rick
I thought I've seen data on the T2FD as made by B&W that at some
frequencies in its rated range the efficiency is only 5% so the dummy
load has to be taking more like 95% of the applied power unless the wire
is dissipating some of that applied power.
73, Jerry, K0CQ
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