Received signals don't just magically find their way to the feedline ...
they induce a current in the antenna that behaves virtually the same as
transmitted signals. Incremental field radiation and incremental field
reception both follow current distribution. Your proposed loop puts a
greater percentage of the current close to the ground ... thereby
raising your takeoff and receive angle because of ground reflectivity.
It's not quite a cloud warmer, but it's heading in that direction.
Think of it this way ... would you intentionally drop your Inverted-V
from 90 feet to 45 feet? No, and the reason you don't is the same
reason you don't want to shift your current distribution closer to the
ground.
With regards to your other comment, size generally doesn't increase
signal-to-noise ratio. It increases the strength of the captured signal
(usually), but it also increases the strength of the noise unless you
get significant directionality improvement out of the larger loop.
Small loops can be effective RX antennas mostly because they can have
sharp directionality for nulling unwanted noise/QRM.
But yes, large loops can be effective RX antennas if they point in the
desired direction. You proposed loop won't (in terms of takeoff angle),
though, at least not compared with your Inverted-V. At least half of
the effective field radiation/reception in your proposed loop is going
to be at a lower height above ground than the current in your Inverted-V
... probably more than half since you're going to have to decrease the
apex angle for your loop.
73,
Dave AB7E
On 12/28/2014 1:06 PM, Rudy Bakalov wrote:
So here's a bit more of my thinking: my main goal is improved RX
performance so does current distribution, presumably a TX
consideration, really matter? The loop is recognized as a low noise
antenna so increasing its size should only improve SNR. No?
Then the other comparison I draw is to small RX loops, which also are
widely acknowledged as effective RX antennas. So what if we make the
small loop big. Really big. Full wavelength big. Is it still effective
as a RX antenna?
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or
inappropriate autocorrect.
On Dec 28, 2014, at 1:21 PM, David Gilbert <xdavid@cis-broadband.com
<mailto:xdavid@cis-broadband.com>> wrote:
In general, there is nothing beneficial about simply having more wire
as long as what you already have is electrically efficient. Yes,
there are some configurations where "more wire" can help ... a loop
high enough off the ground has a db or so gain over a dipole at the
same average height, a curtain array has gain, a dipole extended in
length with capacitance has a bit of gain, etc ... but in your case
your additional wire simply moves more of the current distribution
closer to ground, thereby raising your takeoff/receive angle. It
might be a bit quieter, but not necessarily in a good way ... in my
opinion.
If you want to hear better toward Europe, maybe you could try running
a rope westward off your tower to an anchor point as far from the
tower as you can get it, and then hang a reflector wire off that rope
about 100 feet away from the tower. The reflector would be pretty
close to the ground, but might ... might ... offer some benefit.
Slight improvement in forward gain, modest improvement in rearward
rejection, possibly decent sideward rejection, very slight lowering
of takeoff angle. If probably would alter your current tuning, but
might be worth a try.
73,
Dave AB7E
On 12/28/2014 7:35 AM, Rudy Bakalov wrote:
I actually do have the exact same setup- an inv-V at roughly 90'
with both ends sloping a couple of hundred feet away from the tower.
The apex is more likely ~120 degrees than 90. It works really well
(skimmer data shows actual performance much better than
theoretical), but I was wondering if I can improve on it,
particularly on receive where I feel I don't hear well enough. I
guess my question is "if a piece of wire of certain length works
really well, what happens if I double the length of the wire".
Rudy N2WQ
------------------------------------------------------------------------
I'm curious what you desire to achieve from such an antenna. It's not
likely to be an improvement over what you already have.
A few years ago, before I was able to put up my current tower and yagis,
I suspended a 40m diamond-shaped loop from a 55 foot tall pole ...
sloped toward Europe from here in Arizona and fed at the bottom apex.
It worked OK, mostly because I live on a steep hillside facing Europe,
but in my opinion it didn't offer anything special other than it was
also possible to use it on other bands with a decent tuner. Keep in
mind that unless you feed it at one side for vertical polarization that
half your current distribution is going to be pretty close to the ground.
In my opinion, you'd be better off stringing an Inverted-V from the top
of your tower if you want something horizontally polarized to complement
your existing verticals.
73,
Dave AB7E
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