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Author Topic: Loop for 17m - choices  (Read 3721 times)

Posts: 57

« on: January 11, 2013, 12:23:56 PM »

I'm thinking of trying a wire loop on 17m. I have read alot of information about loops on the net recently (probably too much) so I'm gonna ask for some advice here to try and clear it up in my head. I am primarily interested in DX with this antenna and from what I've read if I go with a 3 sided delta loop feeding it 1/4 wave from the apex on one side with 1/4 wave of 75ohm before the 50ohm coax is the way to go there... but...

Back in the 80s when I was about 12yrs old and on the CB band I made a single wire loop but it was in a diamond shape and fed at the bottom... I was told to do it that way back then and didnt know any different... (obviously I still dont when I am asking this question loll) anyway that antenna worked the world for me with 12w so I was wondering would a diamond shape like that also work on 17m if so where should I feed it and does it still need the same matching stub of 75ohm... or indeed should I be looking at a square quad type loop?

many thanks EI2HPB Smiley

Posts: 97

« Reply #1 on: January 11, 2013, 01:09:32 PM »

I'm running a 17 meter Delta Loop hung under one of my Oak trees. When installed, I fed it as you described and it worked well for DX.   

Recently, I decided to use the loop for other bands (15 & 20), so I removed the 1/4 wave matching section and ran 300 ohm twin lead to a junction box on the side of the house and a few feet of parallel 75 ohm coax from the box to the tuner's balanced input.

I wish I had a tall enough oak for an 75 meter version.

Posts: 17483

« Reply #2 on: January 11, 2013, 01:11:00 PM »

It depends on the relative height above ground and your local ground conditions.

The loop shape really doesn't matter much.  Feeding it at the bottom (or top) gives
horizontal polarization.  Feeding it in the middle of one side (1/4 wave from the top
or bottom) gives vertical polarization.

At very low heights above ground, vertical polarization often gives better low angle
radiation than horizontal polarization.  But if you can get the antenna up high enough
then horizontal polarization will give better results.  The break-even point depends on
your ground conditions (especially those out to 100 wavelengths, far beyond where
most of us can manage to install radials), but in most cases if the bottom of the loop
is up 1/4 wavelength then the horizontal polarization is likely to be better in most
cases.  But if you are operating from the beach next to the ocean, vertical polarization
may be preferable in any case.

So neither approach is "wrong".  Generally the vertically polarized loop is used on 40m
and 80m where antenna height (in wavelengths) is difficult to get.  Horizontal polarization
is more common on 10m where, by the time the antenna is safely out of reach, it is high
enough that horizontal polarization is probably better.  If your support mast is 20m tall
then use one of the horizontal configurations, while if it is only 6m tall then vertical may
be better.

One approach is to put up the antenna using pulleys in the corners, allowing you to pull
the wire and move the feedpoint around the loop to change the polarization.  That lets
you try both polarizations to see which works best for you.

While the antenna shape (diamond, square, triangle, etc.) doesn't have a lot of effect
on the feedpoint impedance, coupling to the ground will vary with polarization.  So at
low heights you may find that a vertically polarized loop has a different feedpoint impedance
than feeding the same shape for horizontal polarization.  Also, as the loop is made
flatter and wider (as is often the case to accommodate low mast heights) the impedance
goes up for horizontal polarization (where the antenna approaches a folded dipole) and
down for vertical polarization (where the antenna starts to look more like a long slot
fed at one end.)

Usually, however, the feedpoint impedance will be between 50 and 200 ohms, and a quick
check of the SWR with direct 50 ohm feed will give you a good idea of the actual
impedance:  if the SWR is, say, between 2 : 1 and 3 : 1 then a quarter wave of 75 ohm
coax should give a good match.

Posts: 328


« Reply #3 on: January 13, 2013, 10:29:11 AM »

I think, if it were me, I would simply put up a Dipole, and forget about the loop.



Posts: 17483

« Reply #4 on: January 13, 2013, 04:03:32 PM »

That's exactly the performance one would expect on 80m:  once you get a horizontal
dipole (or inverted vee) up over 1/4 it will probably work better than a vertical over
typical ground. 

One problem with a vertical plane loop is that half the radiation is at a lower effective
height than the top, because the bottom wire radiates as well.  If you can only get
the peak of the loop up 1/4 wave then the inverted vee is about 3dB better than
the loop at 15 degrees.  But once the antenna is high enough up in the air so that
the lower wire has enough clearance above ground, the loop will give a slight amount
of gain.

A good rule of thumb is that the lower wire has to be at least half the height of the
upper wire before the loop (or any sort of horizontally polarized broadside array) is
going to be better than a single wire antenna at the maximum height.  That's not
always precise, but it seems to be a reasonable estimate, especially for low angle
propagation.  For a diamond loop on 17m, the break-even point (depending on the
required angle of radiation, etc.) is about 40' or so.

There may be other reasons to choose a loop over a dipole, but if you are trying for
low angle radiation, a horizontally polarized loop isn't going to be better than a dipole
until the top is up about 1/2 wavelength or more.
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