Mark Keith wrote:
"Tom Coates" wrote in message ...
The writeup says the terminating resistors make it non-resonant. I wonder
how they affect efficiency.

Seems to me it's already non resonant except on certain frequencies
where it's multiples of a 1/2 wave. But you do have reactance on most
frequencies. The resisters absorb rf that travels along the wire to
the resister. The rf traveling to the rig in the other direction, is
absorbed by that load. So there are no standing waves. Basically, all
the resisters do is make the antenna fairly unidirectional. About the
same deal as a terminated rhombic. You have a good f/b ratio. But
overall total efficiency is appx cut in half, being the waves to the
resister are absorbed. I think this is correct anyway...:/ MK

What Tom probably means is that the resistors turn the antenna into a
traveling wave antenna where the feedpoint impedance is a few hundred
ohms mostly resistive over a relatively wide range of frequencies. It
is somewhat like that infinite feedline that we sometimes talk about.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

Cecil Moore wrote in message ...
Mark Keith wrote:
"Tom Coates" wrote in message ...
The writeup says the terminating resistors make it non-resonant. I wonder
how they affect efficiency.

Seems to me it's already non resonant except on certain frequencies
where it's multiples of a 1/2 wave. But you do have reactance on most
frequencies. The resisters absorb rf that travels along the wire to
the resister. The rf traveling to the rig in the other direction, is
absorbed by that load. So there are no standing waves. Basically, all
the resisters do is make the antenna fairly unidirectional. About the
same deal as a terminated rhombic. You have a good f/b ratio. But
overall total efficiency is appx cut in half, being the waves to the
resister are absorbed. I think this is correct anyway...:/ MK

What Tom probably means is that the resistors turn the antenna into a
traveling wave antenna where the feedpoint impedance is a few hundred
ohms mostly resistive over a relatively wide range of frequencies. It
is somewhat like that infinite feedline that we sometimes talk about.

I guess so. But I think it already qualifies as a traveling wave
antenna on the bands where the wires are actually long enough. My
moment of indecision really came trying to decide if the reduction of
the wave towards the resister qualified as a reduction of efficiency,
or would be a directive loss, IE: like the backside of a yagi. I guess
from reading Roy's post, it does qualify as a reduction of efficiency.
I hope I read that right anyway...But like he says, if you are working
a station in the opposite direction of the resisters, "IE: desired
direction" the loss of efficiency doesn't matter. That lobe *should*
stay appx the same. You are only knocking down the unwanted wave to
the rear. So the gain to the desired station you are pointing to
should be about the same as without the resisters. But the s/n ratio
should improve as you lose the crud and extra noise off the back. Am I
correct here? If not, feel free to jump in... MK

The short answer is, yes, you're correct.

I ignored receiving antennas in my other posting, so I'll say a few
words about them here.

For both transmitting and receiving, the goal is to maximize the
signal/noise ratio at the receive end. But, despite the reality of
reciprocity, the way you go about this in selecting the best receive
antenna is somewhat different than for a transmit antenna. The reason is
that the location of the noise at HF(*) is between the transmit antenna
at one end and the receive antenna at the other. "Noise" can be
atmospheric noise, QRM, or QRN -- anything other than what you're trying
to listen to. To select a transmit antenna to maximize the S/N ratio at
the receive end, you want the antenna that will radiate as strong a
signal as possible in the direction (azimuth and elevation) the signal
will take to the receive station. Period. Your choice of a transmit
antenna has no affect on the amount of noise the other station hears --
it only affects the strength of your signal he hears. So the stronger
your signal, the better the S/N ratio at the other end. If you're
modeling an antenna, just look at the gain in that direction. The more,
the better.

But that's not necessarily true for an HF receive antenna. Consider the
effect of efficiency, for example. If you reduce the efficiency of a
receive antenna in a way that doesn't affect the pattern, both the
incoming signal and noise are attenuated equally. There's no effect at
all on the S/N ratio. Turn up your receiver gain, if you want to make
everything as loud as before, but the S/N ratio won't change. So while
it's desirable to have an efficient transmitting antenna (if efficiency
applies equally to the whole pattern), it doesn't matter with a
receiving antenna. Eventually, of course, you can reach a point where
the incoming noise is so small that the receiver noise becomes audible.
Any reduction of antenna efficiency below that point does impact the S/N
ratio. It's easy to tell if you've reached that level -- just connect a
dummy load to your receiver in place of the antenna. If the noise level
drops, it means that atmospheric noise is still dominating, so your
antenna is adequately efficient. A couple of examples of inefficient yet
effective receive antennas are the Beverage and the AM loopstick.

Another major way to improve the S/N ratio when receiving is to restrict
the antenna's response to the direction the signal is coming from.
Anything coming from other directions is only noise, so if the antenna
doesn't respond to it, the noise is reduced. In the case of a terminated
vee or rhombic, then, the terminating resistors actually improve the
receive S/N ratio by eliminating noise coming from the direction of the
backlobe. (That's assuming, of course, that the station you're
communicating with isn't in that direction.) Nulls in the antenna
pattern in directions of particularly intense noise but not in the
direction of the signal also improve the S/N ratio when listening. When
there are thunderstorms in the Midwest and Gulf Coast, I can hear VK and
ZL stations easily with my 40 meter 4 square array turned to the
southwest that I can't hear with a single vertical. It has nothing to do
with the moderate amount of antenna gain -- the improvement is entirely
due to rejection of the QRN from the thunderstorms.

If you play around a bit with a modeling program looking at two element
arrays, you'll find that the maximum gain occurs when there's quite a
substantial rear lobe. The rear lobe doesn't hurt you when transmitting,
but it can be quite detrimental to S/N ratio when receiving. So most
designers compromise and accept slightly less (actually, an
insignificant amount less) forward gain in order to reduce the rear lobe
and thereby improve the performance when receiving.

If you're considering an antenna for receive use, just remember that
your goal is to maximize the signal to noise ratio. At HF, increasing
the amount of signal invariably increases the amount of noise, at least
from the direction of the signal. So usually, you can help the ratio a
lot more by concentrating on reducing the noise.

(*) At VHF and above, the receiver's internal noise is usually greater
than the atmospheric noise, and this noise isn't between the transmit
and receive antennas. So for most terrestrial communications at VHF and
UHF, the best transmit antenna is also the best receive antenna. One
exception might be the necessity to reject a strong local signal that's
causing intermodulation or receiver overload, placing additional
requirements on the receive antenna.

Roy Lewallen, W7EL

Mark Keith wrote:
. . . should be about the same as without the resisters. But the s/n ratio
should improve as you lose the crud and extra noise off the back. Am I
correct here? If not, feel free to jump in... MK