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Author Topic: HF Multi-Band Delta Loop with AH-4 Tuner  (Read 4075 times)
W4LI
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« on: March 19, 2011, 02:39:51 PM »

Hello,

As I wait for my license to be issued, I've ordered some equipment and am planning a delta loop for multi-band use.  The AH-4 says clearly to NOT use any radiator that is near a multiple of a half wavelength.  I built a spreadsheet and quickly found there aren't many lengths that would work.  However, their instructions relate mainly to radiating elements and a ground/counterpoise.  To what level of precision do I need to avoid the half wavelength, or just exact 1/2 wl multiples?

Although I plan to, I haven't yet dug into antenna theory to answer some questions.  I'd like to get on the air quickly when everything arrives, and then experiment/improve from there.  I am in restricted environment, which limits options somewhat.

I'd like to use a 60-70 foot total length delta loop, corner fed with the AH-4.  The long section would be parallel to the ground and about 4 feet above ground.  The system would be about 15 inches from the house (concrete construction on the exterior walls).  Update: I did adjust and found that 61 feet keeps me 0.04 wavelengths from exact half wavelengths....  at least there is hope.

Since I would be feeding a loop (side to output, bottom to ground; coax rf choke feeding ah-4), I have no sense of whether the end fed voltage problem will arise.  If the radiator length has the same constraints as a normal separate radiator and counterpoise, and I need to stay .05 wavelength away from 1/2 multiples, there would be no working length for even major bands 80-10.

If I do accidentally get too close to this restriction, does it fry the AH-4, or just go into another mode?  If you hop around from band to band, it would be easy to get near the multiple somewhere.

Any advise would be greatly appreciated.
« Last Edit: March 19, 2011, 02:57:44 PM by DANHOOG » Logged

W4LI - Dan Hoogterp
WB6BYU
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Posts: 13143




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« Reply #1 on: March 19, 2011, 03:22:20 PM »

The half wave restriction applies ONLY to end-fed wires.  Loops are different.

The limitation on the tuner is that with a high impedance load (such as an end-fed half wave
wire) the voltage on the antenna is higher for the same power output.  The switching relays
used in most modern autotuners have a limited voltage that they can handle before arcing
over.  If that happens, the relay contacts may get messed up.

With a loop the important dimensions to avoid would be an ODD MULTIPLE OF HALF A
WAVELENGTH.  So 1, 2 or 4 wavelengths would be OK, but not 1/2 or 3/2 wavelengths.

Since you are planning to use this on multiple bands, I'd suggest reading the late W4RNL's
article Notes on All-Band Use of Multi-Band Deltas, available (after registration) here:

http://www.cebik.com/content/a10/scv/vdelt.html

(Registration is free, and there is a huge wealth of antenna information available on the site.)
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W4LI
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« Reply #2 on: March 19, 2011, 03:39:56 PM »

Thanks - that is very helpful and the site looks very helpful as well.  maybe, I'll rethink my first attempt. 

73....  Dan
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W4LI - Dan Hoogterp
WA9UAA
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Posts: 313




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« Reply #3 on: March 19, 2011, 04:58:43 PM »

Dan,
In loops, what you want is a full wave length loop up as high as possible. Granted, a full wave on 80 meters is 1005/ 3.5 mhz. which yields a circumference of 287 feet. The shape can be about any thing you can come up with; though, the greater the area inside the loop the better. A loop cut for 75 meters and using the AH-4 will get you on all the HF bands from 80 meters up through 10 meters. GL HTH
73,
Rob WA9UAA
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K0BG
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« Reply #4 on: March 19, 2011, 06:58:57 PM »

Feeding loops, or any double ended antenna with an AH-4 is problematic.

What it was designed for (AH-4), was feeding a single wire element, against a decent ground plane. In a mobile scenario, that comprises the vehicle body acting as the ground plane, and the element (typically an AH-2) acting as the antenna. In this configuration, you have to be very careful to enhance the ground-side connection of the AH-4.

When you feed a balanced antenna system, like a loop or dipole, you end up having an inordinate amount of common mode current. It doesn't matter that you use a balun or not, it is still there! You end up having to choke the common mode on both the coax, and control lead cables. I might add, that is NOT as easy as it sounds.

If you really want to feed a balanced antenna system, then a decent antenna tuners, with built-in balun, is all but a necessity.

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KI5SO
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« Reply #5 on: June 18, 2011, 03:54:24 PM »

For the benefit of future readers, Just use the Icom AH-4 and give it a try on whatever configuration.  Don't sweat over it.  This tuner works miracles.  It even tunes 160 meters though they do not advertise it to do such.  Just do it.  All the old timey mumbo Jumbo people throw out is ridlious.  Try it and you might be surprised.  Best autotuner on the market and practically does away with the need for coax, except for the length running from radio to the tuner.   ki5so
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