Hi Roger
My TX loop pretty much works as modeled which I can't say for most
verticals I've tries as they are highly dependent on the
installation site, weather and time of year...
My TX loop is a 20m high by 30m long #2 AWG aluminum wire beast. I
find that there is little to no noticable difference in the tuning
during different weather conditions. The loop is installed inside
a line of trees and in some cases in direct contact with the
foliage/branches. I choose the type of conductor I used as it is
tested by CSA to at least 3KV but rated for 600V with a thick XLPE
insulation. Ie. risk of breakdown low and has not been a problem
for me at high power and rainy conditions...
The loop is mounted about 3m over the rocky ground here. I would
not suggest moving the loop base closer to ground and certainly not
burying it. Move it the other way if you can as the higher the
flowing current the higher the ERP. I^2 x R... As you move the
loop closer to ground the losses will go up and the 'I' down... In
fact, I think my poor ground may in fact be the reason for my low
ground losses. My Rloss = 1.21ohms! On high power I can circulate a
current approaching 40A all night long if need be and not need to
worry about the weather...
If someone knows the relationships for how a loop behaves over
ground I'd love to see them too!
My loop seems to work just fine and produces audible signals nightly
at over 2000km range and It has almost completed a QSO with JA
(7200Km) with some nights producing signals in the QRSS10 copible
level there. QSB on the path and poor RX situations (QRM) in JA
being the limiting factors... I can't say that for the vertical
tried at this QTH. Very high feed impedance and it produced a very
poor radiated signal no matter what I did to improve the ground with
the reports or lack of to prove it...
To summarize
Loop - dependable compromise antenna for certain QTH types where
verticals won't work well. Ie. it will work fairly well pretty
much anywhere as long as you have supports for it.
Vertical - If you have good ground/soil and lots of clear land it
will work VERY well. If you're a normal person with a normal lot,
build a loop.
One issue with a loop is as you go QRO the tuner becomes abit more
'complex' to design. Voltages and currents can kill caps. If you
need more info I can supply off list.
73 Scott
VE7TIL
http://www3.telus.net/sthed/argo/
On 9/27/2010 5:24 PM, Roger Lapthorn wrote:
Good to see the reflector back again.
For the last week or so I've been running my vertical "fully in
the air" TX loop (1mm diameter wire, 80m sq area) on both 137 and
500kHz. Now, the bottom of the loop is laying right on the lawn
with no attempt to raise it off the ground. Weather has been mixed
and yesterday it rained all day. Results on 500kHz suggest it is
every bit as good (actually better) than my top loaded Marconi
used last winter with best DX so far with 2mW ERP being LA3EQ last
evening (heavy rain all day and evening). Results on 136kHz with
<20uW ERP have been encouraging too with 6 different WSPR
reports out to 148km. I clearly know that improving the wire
diameter will improve the ERP simply as a result of reducing
resistive losses. What I'm less clear about is why people say all
of the loop should be raised above the ground.
My questions are these:
- What additional losses, if any, do I suffer having the loop
on the ground (even wet ground)? How do these arise?
- Could I bury the return part of the loop wire in the ground
and maybe gain a few more square metres of area?
- Is there anywhere where I can get a simple explanation of
loop losses due to ground presence and foliage presence?
In the last year a lot of accepted theory/beliefs, for example on
earth electrode antennas, has proved to be questionable and I am
wondering if the "loop must be in the air and away from foliage"
is another accepted "fact" which needs to be questioned.
73s
Roger G3XBM
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
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