Topband: Center support on K9AY Loop

[tombaugh]

I've been told that there has been some talk about conductive (metal) center
supports for receiving antennas like the K9AY Directional loop antenna on
this reflector. I have browsed the archives unsuccessfully. Can someone
recall the concensus. Does a metal pole center support degrade the
performance of such loop antennas? Or just point me to the date of archives
and I'll look up the discussion.

Tom Baugh
AE9B

[Phil - KB9CRY]

Yes it will degrade it. Period. Ask Earl Cunningham about it at
***@arrl.net Earl, with others, helped introduce us all to pennants,
loops, and flags.

Post by tombaugh
I've been told that there has been some talk about conductive (metal) center
supports for receiving antennas like the K9AY Directional loop antenna on
this reflector. I have browsed the archives unsuccessfully. Can someone
recall the concensus. Does a metal pole center support degrade the
performance of such loop antennas? Or just point me to the date of archives
and I'll look up the discussion.
Tom Baugh
AE9B
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[Tom Rauch]

Post by tombaugh
I've been told that there has been some talk about conductive (metal)
center supports for receiving antennas like the K9AY Directional loop
antenna on this reflector. I have browsed the archives unsuccessfully.

Hi Tom,

It makes absolutely no difference at all if you use a metal pole or
non-metallic pole as long as you insulate the pole from the wire. The
antenna is a fairly low impedance antenna, and any effect of having a
metal support inside the loop would be immeasurable.. 73, Tom W8JI
***@contesting.com

[Michael Tope]

Tom and Tom:

FWIW, I have been modeling a set of K9AY loops with EZNEC.
My modeling results (real high-accuracy ground - average conductivity)
indicate that their is only a slight difference (about 2dB F/B) at 1.8 MHz
between a grounded metal support, and floating metal support, and
no support (or non-conductive support). The loops I modeled follow
K9AY's suggested dimensions (85' total loop circumference). Modeling
results at 3.8 MHz are similar (minor changes due to mast configuration).

The modeled array was installed locally last weekend using a 30ft
push-up mast that was grounded at the base. Measured performance
seems to be consistent with the modeling on 80 meters (couple of S-units
F/B and improved SNR). The 1.8 MHz measured results don't look
so hot (moderate F/B, but no apparent SNR improvement over the TX
antenna) , but this probably has more to do with the mutual coupling to
the nearby 60' Rohn 45 tower with TH7DX/402CD on top than the
configuration of the array support pole.

73 de Mike, W4EF...............................

[Tom Rauch]

Post by Michael Tope
FWIW, I have been modeling a set of K9AY loops with EZNEC.
My modeling results (real high-accuracy ground - average conductivity)
indicate that their is only a slight difference (about 2dB F/B) at 1.8
MHz between a grounded metal support, and floating metal support, and
no support (or non-conductive support). The loops I modeled follow
K9AY's suggested dimensions (85' total loop circumference). Modeling
results at 3.8 MHz are similar (minor changes due to mast
configuration).

2dB null depth variation is the extreme with the mast near the wire.
It would be even less if the mast were insulated at the antenna by a
few feet of PVC, and less still if the mast were floated at ground.

What many of us miss is how unimportant the peak null depth is in the
ability of an antenna to hear weak signals. What we really want is
directivity, unless we happen to have a very strong problematic
unwanted signal sitting right in the minima of the pattern null. I'd
wager that almost never happens.

The common rumor or idea that a metal mast will noticeably hurt
performance is unfounded, unless you nearly wrap the antenna wire
around the mast. What will destroy the antenna is a poor feed system
or interaction with other structures like towers or power lines. I'd
spend my time worrying about that, and less on things that are
unnoticeable.73, Tom W8JI
***@contesting.com

[Michael Tope]

Any suggestions for how to go about decoupling a nearby tower
from a K9AY loop array, Tom? My modeling suggests that this
may be a problem, but I am not sure if I trust my results. The amount
of pattern distortion shown in the model is a strong function of how
I model the ground connection between the tower and physical earth.
Since I can't make a direct ground connection in EZNEC, I put 8
radials under the tower 4" off the ground. With 8 - 8' radials under
the 45G the modeled interaction with the nearby K9AY loop array
seems to be pretty minimal. When I extend these to radials
to 33' the model shows strong coupling to the K9AY array. With the
long radials in the model, I can minimize the coupling by adding a gamma
arm to the tower and adjusting the series reactance (presumeably this
puts the gamma current in anti-phase with the tower current).

The real performance of the antenna suggests that something might
be going on with regard to interaction with the tower. Although
the arrays shows F/B on 160, it doesn't hear any better than the
TX antenna. I don't know whether this is a function of the
quasi urban environment with the accompanying isotropic noise
(we are up on a mesa with residential/commercial areas visible
over approximate 270 deg azimuth - not much to the north)
or mutual coupling with the TX antennas/nearby tower.
On 80 meters the antenna seems to really work (improved
SNR over TX antenna).

Any suggestions for troubleshooting this situation? I have
a feeling that we are still pretty deaf on topband, so
anything we can do to improve things would be welcomed.
(I would put the array further from the TX antennas, but the
geometry of the available real estate makes this impractical).

73 de Mike, W4EF...................

[Pete Smith]

Post by Michael Tope
Since I can't make a direct ground connection in EZNEC, I put 8
radials under the tower 4" off the ground. With 8 - 8' radials under
the 45G the modeled interaction with the nearby K9AY loop array
seems to be pretty minimal.

Mike, I'm a little skeptical about this technique. Have you tried allowing
the direct connection, but interposing a few-ohm load in the bottom most
segment of the tower (and make that segment very short)?


73, Pete N4ZR
Sometimes a tower is just a tower

[Tom Rauch]

Post by Michael Tope
Any suggestions for how to go about decoupling a nearby tower
from a K9AY loop array, Tom?
model, I can minimize the coupling by adding a gamma arm to the tower
and adjusting the series reactance (presumeably this puts the gamma
current in anti-phase with the tower current).

Sounds reasonable. You probably have severe pattern distortion and
you can probably detune the tower and reduce it. You simply must add
out-of-phase currents in the shunt that are exactly equal and
opposite to the current in the tower. That generally means tuning for
resonance in the gamma if it is a short gamma and if the tower is
less than 3/8th wl long electrical length.

Post by Michael Tope
The real performance of the antenna suggests that something might be
going on with regard to interaction with the tower. Although the
arrays shows F/B on 160, it doesn't hear any better than the TX
antenna.

S/N is improved by the directivity of the array, assuming you have
evenly distributed noise. Some typical directivities:

TX vertical 4.2dB
K9AY, Flag, EWE, Pennant 6-7dB
Two vert 1/8th wl spacing (endfire) 8.5dB
RX 4-square (optimized) 10dB
1wl Bev 9-10dB
pair of 1.5wl 350 ft broadside Beverages 14-15dB

Most situations lean heavily towards directivity setting the
improvement. F/B ratio sounds impressive, but really is somewhat
unimportant unless QRN or QRM is strong and concentrated in the null
direction.73, Tom W8JI
***@contesting.com

[Michael Tope]

Post by Tom Rauch
Sounds reasonable. You probably have severe pattern distortion and
you can probably detune the tower and reduce it. You simply must add
out-of-phase currents in the shunt that are exactly equal and
opposite to the current in the tower. That generally means tuning for
resonance in the gamma if it is a short gamma and if the tower is
less than 3/8th wl long electrical length.

Tom,

Actually there is no gamma arm currently. We are not shunt feeding
the 45G tower (yet). The TX antenna is a low 160 meter dipole
that is sitting at the edge of the cliff another 100' or so in the other
direction (away from the K9AY array). If I add the gamma arm
to the 45G tower (for detuning purposes) how would I go about
adjusting it for minimum interaction? Would it be better to excite
the whole tower parasitically with a nearby whip and transmitter
and then use a current probe loop around both the tower and
gamma arm to adjust the gamma tuning for minimum overall
tower current, or would it be better to excite the tower directly by
connecting the gamma to an MFJ-259 and tuning the gamma for
minimum reactance on the impedance analyzer?

Thanks!

Mike, W4EF...............................

[Michael Tope]

According to the EZNEC manual, whenever you insert the direct
connection with the Sommerfield ground, NEC inserts an unknown
series resistance. If I switch to real mininec ground, I don't get any
warning (according to the manual, this is the preferred ground for
vertical elements with direct connections to ground). The results
with the direct connect to the real mininec ground look reasonably
close to the Sommerfield results with the direct connection (so perhaps
the warning with the sommerfield ground is unwarranted in this
particular case).

Anyway, too many variables and question marks with the computer
models in close to ground to give me much confidence in the
Nth decimal place results (its all giving me a headache). At this point I
suspect the best thing to do is abandon the computer and do the
final tweaking on the array (and the tower detuning) in the field
with real measurements. Besides I can't work any DX with the
computer model no matter how good I make the patterns look.

73 de Mike, W4EF............................

[Donald Chester]

Looking over both QST articles, it seems to me that the K9AY loop is nearly
identical to the diamond or delta version of the flag/pennant. The main
difference is the location of the feed point and termination resistor, the
termination resistance, and the grounding point with the K9AY loop. The
horizontal space is nearly identical (about 30 ft.), but the K9AY loop
requires a taller support (about 25 ft. versus about 15 ft). I may
construct one of these antennas, but instead of two loops 90 degrees from
each other and a complicated switching scheme, I think it would be simpler
to make the whole thing mechanically rotatable. The dimensions don't seem
prohibitive, and that way you could could find the maximum null for local
noise or QRMs much in the same manner as with the well-known rotatable
shielded loop.

Any comments on how these two antennas compare in performance, by someone
who has actually constructed and tried both?

Don K4KYV



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