Topband: K9AY Loop Revisited

[Michael Tope]

> Hello All:
>
>
> I broke down and ordered a K9AY loop kit from Array Solutions today. I
know
> there has been some discussion lately on receiving antennas,
> but I'm looking for any helpful tips.  I did order the fiberglass mast, so
> the metal mast ? is solved.
>
> 1.  Can I use high quality RG-58?  The coax run will be about 150 feet. My
> gut feel is yes (low loss at 1.8/3.5 mHz), but does it lack the proper
> shielding for anti-noise pick-up?
>
Yes. At 150' your feedline loss will be small, even with RG-58, so
it shouldn't make too much difference. You may need a pre-amp
depending on your rig. At the local club station where we have a
K9AY loop array, I have noticed that the output level is a wee bit
low for my liking on 160 meters. The noise floor from the loops stilll
overcomes the internal noise of the rig, but the resulting audio level
(even at max volume) doesn't quite get it with my old rock'n'roll
hardened ears. YMMV. On 3.5 MHz, the output level is high enough
that listening is comfortable without a preamp.

If you have any doubts about the shielding effectiveness of RG-58
at 1.8/3.5 MHz. Terminate 150' of coax into 50 ohms and then
plug it into you receiver. If you don't hear anything then you'll
have your answer. I would expect that any kind of RG-58 with
reasonably good shield coverage will provide adequate shielding
at 1.8/3.5 MHz.

> 2.  What's preferred, being all things somewhat equal.  Mounting the
antenna
> 40+ feet from an overhead powerline (house drop -- 22 foot high mast +
> safety factor) or mounting farther away from the power line, but closer to
> my Hy-tower? (I run 150 watts).
>
The Hy-tower is guaranteed to be resonant at your operating frequency,
whereas its probably a crap shoot with the power line. On the other hand,
if the power line is resonant and couples strongly to your loops, you will
have a tuff time detuning it whereas you can easily come up with a scheme
to detune the hytower on receive using some relays. If you do go close to
the hy-tower, I suggest you use somekind of receiver protection (like the
KD9SV front-end saver) to open or short the loop output during transmit.
This
is what was recommended to me by a number of people on this reflector
when I asked recently.

BTW, detuning of nearby objects (like the Hy-Tower) appears to be
very important. Our loops work great on 80 meters where my full
station computer model shows that nearby antennas don't couple
strongly. On 160, the computer model shows that the nearby tower
is coupling strongly to the loops and sure enough the loops don't hear
any better than our transmit antenna on 160. Hopefully we'll get that
fixed in the next couple of weeks.

> 3. What's the final verdict on the RX antenna jack on the FT-1000MP MK5?
Is
> it safe to use or is a separate switch box needed?
>
Dunno about the 1000MP MK5, but the FT-1000D doesn't appear to have any
protection on its external receive input (at least none that I could find
on the schematic).

> 4. What type of ground does the K9AY loop need?  The installation manual
> says 4 or more 10 foot ground radials will help, other K9AY loop articles
> I've read says ground loss is part of why the antenna works.
>
We have 3  15 foot radials and a ground rod on ours. This seems to work.
BTW, the part about ground loss being necessary isn't true. The K9AY
loop is basically an approximation to a pair of closely spaced short
phased verticals. The ground loss will effect the optimum termination
resistance, but it isn't a requirement for good performance (the K9AY loop
should work fine over highly conductive ground). It is the beverage antenna
that relies on ground loss to work properly.

> Any help appreciated.  BTW, something must be working here, I worked my
> first VK on topband two weeks ago using my Hy-tower with the MK-160
inverted
> L adaptor.
>
> Greg
> K0PJ/9
>
>
>
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