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Author Topic: Large horizontal loop configuration  (Read 1024 times)
KI6NUJ
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« on: April 10, 2008, 11:56:15 PM »

I am planning to setup a horizontal multiband loop on my apartment building's flat roof. It is roughly 60ft x 25ft.

Questions:
1. The roof's perimeter of about 170ft is short for a full wavelength 80m antenna and longer than one wavelength 40m antenna. So should the antenna length be cut to one wavelength at 40m or should I use up all the available 170ft?

2. To feed the antenna with a coax and avoid having a tuner at the feedpoint, is it ok to have a 1:1 current balun at the feedpoint, then a run of about 75ft of coax and then to the tuner?

3. Is it ok to use the built-in tuner of the Kenwood TS-570DG? If not, what kind of external tuner is better - manual or auto?

4. How does an auto tuner determine a good match? By only VSWR or VSWR and RF current? If the answer is only VSWR then is a manual tuner with a RF ammeter a better combination?

73,

- Siddhartha
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W8JI
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« Reply #1 on: April 11, 2008, 03:29:50 AM »

I would cut the loop for 40 meters, and try to have a way to open the loop exactly opposite the feedpoint for 80 meters. That might require a manual jumper or a good vacuum relay.

If you can make the loop resonant feeding it with a  1:1 balun and coax would be fine.

The tuner in the rig will be fine if you operate somewhat close to the resonant frequencies.

73 Tom
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W8JI
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« Reply #2 on: April 11, 2008, 03:31:09 AM »

By the way, SWR is every bit as good an indicator as antenna current. Auto tuners sense SWR. I can't see any reason to measure antenna current.
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WA3SKN
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« Reply #3 on: April 11, 2008, 04:31:25 AM »

I assume you want multiband HF operation.
The built-in tuner is designed to match a 3:1 VSWR max, it will not match this!  Turn it off.
I would go ahead and use the maximum loop you can install and feed with open wire line and a manual tuner.  But, using coax and having an auto-tuner at the feed point should work... you will want a balun.
Most auto-tuners do not handle the extreme impedances a manual will, but are very convenient... and fairly fast.
You can always make the loop smaller, if needed.
73s.

-Mike.
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WV4L
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« Reply #4 on: April 11, 2008, 05:07:23 AM »

I concur with Mike. Use up the available space you have and feed it with ladder/open wire down to a balun with a short run of coax(no more than 15-20 ft.) to an external tuner. This will give you multiband capabilities on most bands. Have fun!

73

Wayne
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W8JI
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« Reply #5 on: April 11, 2008, 06:26:44 AM »

Why would or should he use the maximum space available when it will add feedline loss, complicate matching, and not make the antenna itself work any better?

I'm just looking for a logical reason to intentionally mismatch the antenna by making it non-resonant.

The only real advantage of a large closed loop is it has a modest impedance on all harmonics of the full wave resosnant point. That's the sole advantage. When someone makes it non-resosnant that advantage disappears.


So why would someone want to do that when it complicates the feed system, adds feeder loss, and doesn't make the antenna itself work any better?


What is the logic behind that?

73, Tom
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K4SAV
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« Reply #6 on: April 11, 2008, 06:41:38 AM »

Yeah, you can make it full size and feed it directly with coax, and you can probably contact someone, but the loss in the coax will be very high.  An EZNEC estimate shows the loss in that 75 ft of coax to be between 9 and 22 dB depending on the band.  The 22 dB was on 80 meters.  Most of the other bands run around 11 dB loss.  W8JI gave you the best suggestion if you have to use coax.

Jerry, K4SAV
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WA3SKN
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« Reply #7 on: April 11, 2008, 07:42:19 AM »

Use open wire to the tuner located by the radio if at all possible, try to leave coax out of the system.
It will load OK from 1/3 wavelength, and will radiate well from 2/3 wavelength on up.  With a manual tuner, matching should not be a problem.
73s.

-Mike.
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WB6BYU
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« Reply #8 on: April 11, 2008, 08:39:51 AM »

If you are trying to be discrete about this installation, sometimes the
best thing to do is simply to put up as big of a loop as you can and
feed it with twinlead or open wire line to a tuner in the shack.   This
is especially true if you are hanging it around a roofline, for example,
where you don't have much control over how long it can be.  But such
a random-length loop will be very inefficient if fed with coax to a tuner
in the shack.

If you can make the loop resonant on 40m it will have a relatively
low SWR on 20, 15 and 10m also, in which case coax feed probably will
work well enough.  (I would typically use a 4 : 1 balun at the feedpoint
which would give a better match on the higher bands, unless my main
interest was 40m.  Which is often is.)

Note that you can't rely on the loop length formulas when mounting an
antenna on a roof.  We put up a 40m antenna on a composite roof once
and it resonated at 5.8 MHz.  I generally assume I have to shorten the
loop at least 10% when it is laying on a roof or hung behind the fascia
board.  It may take you a few trips to the roof to get it resonant where
you want it.  If this is too conspicuous (or dangerous) then the random
length loop may be more practical, as long as you realize that coax is
NOT the best feedline to use for it.
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W8JI
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« Reply #9 on: April 11, 2008, 09:53:14 AM »

The only real electrical advantage of a loop is the periodic repeating of a low impedance on harmonics of a full wave.

If I had a roof that only allowed a 40M loop plus a little, I'd either make it a full size on 40 so feed impedance was modest on 40,20, 15, and 10. This would greatly simplify getting the feeder into the house. Or I'd leave the loop OPEN and make it a half wave on 80 and use a ladder line and good tuner.

It makes no sense at all to make the loop "as big as possible" when it is too small for 80 when closed, and too big to hit four primary bands. Making something worse across the board just so it is "bigger" is very poor planning.

Note no one can state an advantage. Just a feeling that bigger is better.

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AD5ZC
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« Reply #10 on: April 11, 2008, 10:24:12 AM »

I vote cut it for 40 (140 ft) and make it as square as possible for the most omnidirectional performance.

Like Tom said, If you can incorporate a relay opposite the feedpoint to open the antenna and make it an odd shaped 80 meter dipole thats the best way to try to get 80 out of this antenna.

You definately want to use balanced feeder and a good tuner.  
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KI6NUJ
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« Reply #11 on: April 11, 2008, 01:40:38 PM »

I was beginning to side with W8JI. Why have a bigger than one WL antenna that you anyway need to tune down to the electrical length of a one WL?

But WB6BYU makes a good point especially wrt my requirements where the antenna is going to lay on the roof, probably inside a PVC pipe rectangle. I will probably start with 170ft and trim it down till I get it resonant on 40m.

About the tuner - How do I feed the balanced line into the auto tuner of the xcvr? Or, is it better to buy a manual tuner that takes balanced line input?

The reason I ask about the ammeter is that I read somewhere that a tuner will offer several combinations where the SWR is the same but you need to shoot for the combination that gives you maximum RF current. Apparently, this was a standard practice in the years gone by. True?

Thanks,

- Siddhartha
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K1BXI
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« Reply #12 on: April 11, 2008, 04:23:40 PM »

"I read somewhere that a tuner will offer several combinations where the SWR is the same but you need to shoot for the combination that gives you maximum RF current. Apparently, this was a standard practice in the years gone by. True? "

True, but in those years gone by, open wire feeders and link coupled atu's were the norm. The only practical way to tell if the tuner was "tuned" was to measure the rf current in each side of the feed line, usually with rf amp meters.

You will find with your C-L-C tuner that the best match and power transfer is with the largest C that shows the lowest SWR. Use a field strength meter with an outside pickup antenna to verify this if you still have any doubts.

John.....K1BXI
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WV4L
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« Reply #13 on: April 11, 2008, 04:53:25 PM »

About the tuner - How do I feed the balanced line into the auto tuner of the xcvr? Or, is it better to buy a manual tuner that takes balanced line input?

That's where the balun comes into play. The open wire line comes down to the balun which you connect your coax to. That's why you want to keep the coax run short, so the losses are minimal. The losses on the open wire feedline are minimal to start with. Look at the line losses on one of the tables in the antenna book or in the antenna section of the Extra Class manual.
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W8JI
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« Reply #14 on: April 11, 2008, 06:23:07 PM »

What type of feedline can you install and how long will it be?

Remember open wire line needs to be a few conductor spacings away from anything metallic or that might be a lossy dielectric. A one inch spaced line needs to be a few inches away from anything that can affect it.

What I might do would really depend greatly on what I COULD do with the antenna. Since no one knows that, no one can really give you accurate advice.

The conditions I gave were for feeding the antenna with coaxial line or even open wire of a random length. I also assumed since you were in an apartment open wire or ladder line, especially of some "sweet length", would be a major headache. It would be very visible and have to be kept well away from metal or things that get soaked. Also if you use ladder line certain lengths can make some very weird impedances if the antenna is not reasonably close to being resonant.

Say for example you take some advice and make an antenna that is not resonant. The feedpoint SWR might be 40:1 referenced to the line impedance with an antenna cut between 80 and 40 meters. With 400 ohm line (the typical real impedance of ladder line called 450 ohm) the input impedance could be as low as 10 ohms or as high as 16,000 ohms.


You can see why I recoil at suggestions you can just make the antenna any odd length if you use ladder line. That simply is not true, unless you are willing to tolerate or handle other problems you will create. It is easy to accidentally create a system that you cannot match.

It is ALWAYS better to plan the length. It is ALWAYS better to have a reasonable SWR on a transmission line, even if the line does have very low loss. Otherwise you have to plan the length of the line, or determine it by cut and try Edisonian methods.

The primary advantage of a loop is the period repeating of the modest feed impedance on harmonics. It is just silly to toss that advantage out at a whim for no reason at all.

73 Tom



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