[Antennas] Query (delta loop vs inverted vee)

[Terry Conboy]

At 09:27 AM 2008-01-01, Chris Boone WB5ITT wrote:
>Right now, our local club has a delta loop....top is about 70ft off ground,
>bottom part of maybe 20ft off ground; slopes maybe at a 30 degree angle down
>from vertical...overall length is about 250ft (all three sides). Fed with
>RG8 to tuner in the clubhouse (about 100ft or so). I feel it would be better
>as a inverted V instead of a closed loop. I feel the currents so close to
>the ground on the bottom section are hurting more than helping but I dont
>have a copy of any decent antenna software to prove it. BTW we are wanting
>to run this antenna on 160-10...right now 160 is a lost cause. The tuner
>wont get anywhere close...Even 80m is touchy and doesnt radiate worth a fip
>(of course the MFJ tuner may be responsible)...

At 09:48 AM 2008-01-01, Chris wrote:
>Fed at the top...no balun (not that I can see).....

Here's a comparison azimuth plot of the two at a 20 degree takeoff 
angle on 75m, showing the inverted vee has about 1.5 to 2 dB more gain:

         (I sent Chris the four plots, of you want them, reply off list)

Here's an elevation slice on 75m broadside to the plane of the wires, 
which shows about 1 dB or less advantage for the inverted vee at higher angles:

         (elevation plot by request)

Although the loop has slightly less gain (for just the reason 
mentioned), there are some other advantages for multiband 
operation.  An 80m inverted vee will have a very high feedpoint 
impedance on 40m and up (except 30m), making coax feed 
impractical.  (Of course, with a good balanced tuner, an open wire 
feed would work fine.)

On the other hand, the loop will have modest impedances at harmonics:
200 ohms at 3.98 MHz
120 ohms at 8 MHz
240 ohms at 11.95 MHz
340 ohms at 15.7 MHz
160 ohms at 19.7 MHz
It would be easier to match if the loop were a little longer, moving 
the resonance down from 3.98 MHz so that the harmonic resonances fall 
closer to the higher bands.

If your club is more DX oriented, it would be quite beneficial to 
feed the loop near the bottom of one of the sloping sides (~1/4 wl 
from the top).  This makes it vertically polarized and much more 
effective at low angles (under about 25 degrees).  The loop has 1 to 
2 dB more gain than the inverted vee at 20 degree takeoff:

         (azimuth plot by request)

You do give up a lot of NVIS signal, however (which can be good or 
bad, depending).  Below 20 degrees the side fed loop should 
outperform significantly:

         (elevation plot by request)

Side feed also changes the feedpoint impedance, but it still will be 
reasonable at harmonic resonances:
72 ohms at 4.13 MHz
117 ohms at 7.96 MHz
223 ohms at 11.9 MHz
325 ohms at 15.8 MHz
395 ohms at 19.73 MHz
(Note the shift in resonance on 75m with side vs. top feed).

Regarding 160m... it is possible to feed an 80m inverted vee on 160m 
with open wire feed, but the impedances are a bit crazy (about 6 - j 
1250 ohms at the feedpoint) and many tuners won't deal with this 
unless you prune the line length (if then).  This represents a SWR of 
over 100 on 450 ohm line!  Line loss WILL be an issue.

You can feed a closed 80m loop on 160m, where it is anti-resonant (a 
VERY high impedance with low reactance).  It is possible to use a 1/4 
wl (on 160m) open wire line (the higher impedance the better) to 
transform the high feed Z to a low value that most tuners can 
accommodate.  Despite the low load reactance, the line losses, even 
with high quality open wire line, can be several dB.  Note that a 1/2 
wl loop has the current maxima at the point opposite the feedpoint, 
which would argue for the use of bottom center feed if 160m operation 
is considered with a closed loop.

A possible compromise is the use of a double-size G5RV (approx 204 
feet overall) in an inverted vee configuration.  There isn't any 
magic to this, but the impedances are manageable with most tuners on 
most bands, including 160m.  If necessary, the ends could be bent in 
or to either side to fit in your space.

Hope this helps.

73, Terry N6RY