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Author Topic: Horizontal Mobile HF Loop  (Read 2305 times)
KK7XE
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Posts: 4




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« on: March 14, 2017, 08:15:51 AM »

Planning to build an HF mobile loop out of 1/2" conduit. It will be 1' above the cab of my pickup, eliminating any interaction from cab. The feed line will come from a 6" gap that will be by pvc down to an Icom AH-4 tuner. The antenna will be supported by PVC pipe at the 4 stake holes. It should hold up well and have had a great deal of experience using the AH-4 in a variety of base antenna set ups.
Has always been a great tuner with my various antennas. The tuner will be in a NEMA enclosure. Think it will be a fun project and am looking for any input on my idea. Thanks for reading.
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KK4YDR
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Posts: 673




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« Reply #1 on: March 14, 2017, 05:03:39 PM »

Ive toyed with the idea of an RX loop but in the mobile it would be absolutely no xmit. A tuner is horrible for a mobile xmit antenna but would be required for an rx loop. If you have the space I say go for it.

Unfortunately you cant isolate something like that from the massive mass of steel and Al under the antenna. PXmit is goinf to be a disaster but RX could work very well.
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K1TWH
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Posts: 115




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« Reply #2 on: March 16, 2017, 08:22:35 AM »

Read around on small loop articles.   They typically require a specialized matching network and on a good loop a 7MHz bandwidth might be 3-5 KHz wide (2:1 ~SWR).   The AH4 is a good unit, but a 0.1 ohm input impedance with a ton of reactance is going to result in a lot of lost power in almost any tuner not specifically designed for a loop.   You could investigate the MFJ-935/6 loop tuners and / or download the manual to get a feel for how they match a small wavelength loop.     -Tom
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WB6BYU
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Posts: 17181




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« Reply #3 on: March 16, 2017, 10:21:39 AM »

A few thoughts:

Copper pipe will have lower losses than steel conduit.  Steel is comparatively lossy at
RF due to the hysteresis losses (the magnetization / demagnetization of the metal for
each RF cycle.)  This isn't too much of an issue with a dipole, but a loop will have VERY
high currents with low radiation resistance, and conductor resistance plays a larger
role in antenna efficiency.

What bands do you plan to use it on?  What distances do you want to cover?  A small
loop has a null overhead with maximum radiation off the edges.  It won't be very good
for NVIS coverage on 80m or 40m because of the overhead null.  (And higher frequencies
don't support NVIS propagation.)  Basically you are using a horizontally polarized antenna
that is about 7' high, which won't have a lot of low angle radiation either, except perhaps
on 10m.

How short of a wire is the AH-4 rated to match on the bands where you plan to use it?
Short end-fed wires have a low resistance and a lot of capacitive reactance.  Loops
have a very low resistance and a lot of inductive reactance.  Don't be
surprised if the AH-4 has problems matching the loop.

And using any length or type of feedline between the loop and the tuner may be
problematic.  Or at least lossy.

A foot of clearance over the top of the cab is nowhere near enough to reduce the
interaction between the antenna and the truck.  Even just the bed of the truck
likely will be strongly coupled to the antenna, and may contribute to the losses.
Consider the typical spacing between a driven element and a reflector in an
antenna for the bands you are planning to operate - even that provides strong
coupling between the elements.


That's not to say that it isn't worth trying your idea out to see how it works.
There is a lot to be learned by trying new things, even if (in fact, especially
when
) they don't go as planned.  It certainly would be interesting to
be able to switch back and forth between the loop and a typical mobile whip
to compare signal strengths on different stations.


Quote from: Mark Twain

If you hold a cat by the tail you learn things you cannot learn any other way.

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KA4NMA
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Posts: 518




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« Reply #4 on: March 20, 2017, 12:02:46 AM »

Check out the SGC Auto couplers and SGC work.com

Randy Ka4nma
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