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Author Topic: Shorter or Longer to Alter SWR?  (Read 4926 times)
KC2NYU
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Posts: 136




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« on: August 14, 2012, 04:46:51 AM »

Antenna theory has never been a strong suit of this history major, so need some advice from the forum experts.
-I put an Inverted L using single strand 14 ga insulated wire. Goal was to get on 160M, so dimensions are 62ft vertical and 68ft horizontal. Vertical runs up pine tree, horizontal runs thru one tree and is secured to another tree. Have two 65 ft above ground radials (3 ft above ground), one runs under the horizontal leg and the other runs off in opposite direction. Right now both the SWR and the Impedance is too high.
-   So here’s my question --- do I shorten or lengthen the wire to lower the SWR and Impedance? Depending on the answer, do I also modify length of radials?

Tnx & 73
Paul
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AA4PB
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« Reply #1 on: August 14, 2012, 05:10:34 AM »

Check the SWR at different frequencies. If the SWR goes down as you increase frequency then the antenna is too short. If SWR goes down as you decrease frequency then the antenn is too long.
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K9IUQ
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Posts: 1954




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« Reply #2 on: August 14, 2012, 05:50:52 AM »

Depending on the answer, do I also modify length of radials?
Tnx & 73
Paul

I use a inverted L similar to yours except mine goes up vertically 60ft about 4ft from my tower. I have about 35 radials in the ground. The radials are about 130ft long and lots of them do not go in a straight line. It has been my experience the more radials I put down the higher the SWR. When I first started this antenna I had only 4 radials. The SWR was pretty good around 1.2:1. I started adding more radials and the more I added the worse the SWR got. At presently with 35 radials the SWR never goes below 2:1 and goes higher depending where in the band I am. I have tried to tweak the length of the L but it does not help much.

I do not worry about it. My antenna tuner tunes the L to 1:1 and it works great.

My advice: Add as many radials as you can and if the tuner can tune the antenna to a SWR your radio can handle do not worry about it.

Stan K9IUQ
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AA4PB
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« Reply #3 on: August 14, 2012, 06:19:01 AM »

Radials may be part of your high SWR problem. Each above ground radials should be 1/4 wavelength long (128-feet on 160M). Your 65-foot radials will present a fairly high impedance at the antenna feed point. If you don't have space for longer radials then put them on or in the ground so they are non-resonant and use as many as possible - even if varying lengths or bends are necessary.
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W8JI
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« Reply #4 on: August 14, 2012, 07:22:25 AM »

Antenna theory has never been a strong suit of this history major, so need some advice from the forum experts.
-I put an Inverted L using single strand 14 ga insulated wire. Goal was to get on 160M, so dimensions are 62ft vertical and 68ft horizontal. Vertical runs up pine tree, horizontal runs thru one tree and is secured to another tree. Have two 65 ft above ground radials (3 ft above ground), one runs under the horizontal leg and the other runs off in opposite direction. Right now both the SWR and the Impedance is too high.
-   So here’s my question --- do I shorten or lengthen the wire to lower the SWR and Impedance? Depending on the answer, do I also modify length of radials?

Tnx & 73
Paul


Paul,

AA4PB gives good advice. Your radials are not even close to what you assume they are. They are far too short.

An elevated counterpoise is a very special case. The antenna connection point of the elevated counterpoise, even if it had 1/4 wave radials, has considerable voltage to earth. In your case, because the radials are so short, it has huge voltages to earth.

This means the shield or anything you connect to the radials has considerable RF voltage to earth and other things around the antenna. If you run a ground rod to the counterpoise, your signal level will decrease. If you don't decouple the coaxial feedline, you will bring RF into the shack and noise out to the antenna.

Also, since the voltage at the feedpoint on the counterpoise is out-of-phase with current, the ground looks reactive. This capacitive reactance will make the antenna "look shorter".

You need to decide how to fix the counterpoise, and you absolutely need to isolate the coax shield from the common of the counterpoise with some type of feedline choke. There are several ways to fix this, but the first step is always in recognizing what is really wrong.

1.) ANY sparse counterpoise with less than a few dozen adequate length radials has considerable voltage to earth, and needs decoupled from earth. The smaller the counterpoise the more critical.

2.) Any non-resonant small counterpoise will detune the antenna, and will often change the feed impedance drastically.

Those are the two problems you need to fix *****BEFORE***** adjusting antenna length if you want a system that works properly and reliably.

I have started some stuff here:

http://www.w8ji.com/fcp_folded_counterpoise_system.htm

Pay particular attention to counterpoise voltage to earth at 1500 watts.

73 Tom
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K9IUQ
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Posts: 1954




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« Reply #5 on: August 14, 2012, 07:39:03 AM »

you absolutely need to isolate the coax shield from the common of the counterpoise with some type of feedline choke.
73 Tom

I used a homebuilt Collins choke made with coax when I first made my inverted L. Later I bought a nice one from Dx Engineering. It worked so well I bought a second choke for another vertical I have.

Stan K9IUQ
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KC4MOP
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Posts: 743




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« Reply #6 on: August 14, 2012, 08:03:31 AM »

You will probably have to go to STEP 1 and decide how long or short your antenna is now. I hope you have the MFJ 259 or 269. By adjusting the freq of the analyzer you will see where or IF the SWR decreases.
The IFs of a new antenna system:
If the analyzer starts showing resonance at 3mhz (exaggerated) then the antenna is too short. The horiz wire.
IF the analyzer ( seriously doubt this) is showing resonance at 1700khz then the antenna is too long.

Before Step 1 you will have to get more radials out there. Not to discourage you; but ground mounted radials would be a nice starting point for a new antenna installation. Run about 30 of them as long as you can afford to lay out there. THEN start to look for any resonance of your antenna.
Typically I have read 60 some feet vertical and possibly 100 feet horiz about the same height, if you can deal with this. So, you are almost there.

Elevated radials open another "can of worms" but a starting point would be 120 feet long, 10 of them raised about 10 feet from the Earth. This is a starting point, elevated radials have to be tuned to be effective. It is not a ballpark thing. 4-10 raised radials are more effective than 120 radials as the AM broadcast folks do.

Keep us in the loop on your progress. Antenna season is here now as Summer is ending.
Fred
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KC2NYU
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Posts: 136




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« Reply #7 on: August 14, 2012, 08:09:57 AM »

First - thanks for all the good advice. which raise more questions and a few comments:

- first reply fm AA4PB, may have a typo, in both sentences it says If SWR goes down, should one be "goes up" ?_______________________________________
Check the SWR at different frequencies. If the SWR goes down as you increase frequency then the antenna is too short. If SWR goes down as you decrease frequency then the antenn is too long.


-- I can make thetwo above ground radials longer; but my lot backs up to a golf course and have small yard so in ground radials not easy to do.

-- I have a 1:1 balun , would that work as a feedline choke ?

-- I have two 8 ft ground rods driven at the base of the tree. I currently just using to connect to lighting arrestor on feedline. Had been told previously that radials should not be tied to ground, and as Tom says that would serve to decrease signal.

73 Paul
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W8JI
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« Reply #8 on: August 14, 2012, 08:12:47 AM »

Are you aware the radials need to be somehow tuned to 160 meters??? Did you catch that point?

Yes, a GOOD 1:1 current balun will serve as a feedline isolation device. You still need to get that radial impedance down to make things easier on the balun, and make it easier to tune the antenna.
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KC2NYU
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Posts: 136




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« Reply #9 on: August 14, 2012, 08:24:53 AM »

--- I do have an MFJ antenna analyzer. So far I just checked 160M, and the SWR at 1800khz was 10.5, at 1900 was 6.5 and at 2000 was 3.6. I will check other bands.

---- How do I " tune" the radials to 160M  ?

73 Paul
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KC4MOP
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Posts: 743




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« Reply #10 on: August 14, 2012, 09:16:39 AM »

--- I do have an MFJ antenna analyzer. So far I just checked 160M, and the SWR at 1800khz was 10.5, at 1900 was 6.5 and at 2000 was 3.6. I will check other bands.

---- How do I " tune" the radials to 160M  ?

73 Paul

OK, by your original dimensions, the antenna is too short. Lengthen the horiz wire A LOT! Best to start long and start trimming.
Your analyzer will be the main tool for 'tuning' the radials. And I am not able to give a reply to that. Someone will give the correct procedure, then I will know.
Other bands....hmmmm you will need to have a tuning network at the base of your "L" and that will not go well with your tuned radials for 160M. You will need more tuned radials, same number, for each band. See where the elevated radials is heading??? Inconvenience at your QTH. and the golf course.

The "L" antenna is a great antenna for 160-40. 40M might be pushing it for pattern distortion. The way your radio signal leaves your antenna to get into the atmosphere and skip to a distant location.

If I may suggest that you simplify your life and lay down as many radials on the ground as you can. 30-40 as long or as short you can fit on your property. Laying anything on the golf course would not go well with the owner or the players.
What are the dimensions of your property? How would the idea of chicken fencing sound as part of a ground radial system. Just trying to minimize the losses of your radio signal (TX) from going into the Earth.
Some Hams in the marshy areas of the USA. Mississippi and Alabama have super ground conductivity and do not need ground radials. You can check on your QTH with a simple Goggle search. It is probably like the most of us Ham radio guys.

http://www.fcc.gov/encyclopedia/m3-map-effective-ground-conductivity-united-states-wall-sized-map-am-broadcast-stations
You can download what looks like a 100 tiny maps of the USA and see your ground conductivity.

Are you still in Ga.? That looks like fair/typical ground conductivity. I live in Western Pa. and we are about the same.
I'm chattering too much.....the size of your property is going to dictate how much a ground screen you can lay down. The chicken wire fencing will create a concentrated screen to complete the second path for your RF (which is A.C.) for a return.
Thanks
Fred
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AA4PB
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Posts: 12891




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« Reply #11 on: August 14, 2012, 09:34:08 AM »

Check the SWR at different frequencies. If the SWR goes down as you **increase frequency** then the antenna is too short. If SWR goes down as you **decrease frequency** then the antenna is too long.

The statement is correct. However, you are wasting your time trying to adjust the antenna length before you fix the radial problem. Initially I didn't read close enough to notice that you are using radials that are no where near resonant.

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KC2NYU
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Posts: 136




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« Reply #12 on: August 14, 2012, 09:53:42 AM »

Robert-
- my bad, I didn't read your statement carefully- now understand.
- roger wasting my time until I fix radial problem.
- so I need to extend each of the radials from 65 ft to 128 ft.
- earlier post said I need to "tune" the radials-- how do I do that?

will work on radials and then report back.

Tnx and 73
Paul
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AA4PB
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« Reply #13 on: August 14, 2012, 10:56:59 AM »

I think that cutting them to 128 feet (a calculated 1/4 wavelength) will probably be close enough. Having them near the ground is going to detune them somewhat anyway. A better way would probably be to put a bunch of radials on or in the ground (just below the surface) and run them in all direction in whatever lengths fit the available space. Twenty 64-foot burried radials will be better than two 128-foot burried radials. With burried radials you are attempting to improve the conductivity of the soil. With suspended radials the goal is to make them exactly 1/4 wavelength long with the far end "floating". The floating end is high impedance so 1/4 wavelength back (at the feed point) is low impedance.
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KC4MOP
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« Reply #14 on: August 14, 2012, 02:52:56 PM »

I think that cutting them to 128 feet (a calculated 1/4 wavelength) will probably be close enough. Having them near the ground is going to detune them somewhat anyway. A better way would probably be to put a bunch of radials on or in the ground (just below the surface) and run them in all direction in whatever lengths fit the available space. Twenty 64-foot burried radials will be better than two 128-foot burried radials. With burried radials you are attempting to improve the conductivity of the soil. With suspended radials the goal is to make them exactly 1/4 wavelength long with the far end "floating". The floating end is high impedance so 1/4 wavelength back (at the feed point) is low impedance.

I think we are trying to ask you to RE-think elevated radials and go with as many as you can, however long they can be, on the ground radials. 60 something radials is pretty close to what you need.
I don't think KC2NYU has enough space for 128 feet long radials. He hasn't given any dimensions for his property.
TUNING the radials was from your original post. You said you wanted elevated radials. Several of us are trying to steer you away from elevated to just good ole ground radials. Whatever you can lay out there.

Fred
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