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Author Topic: fluctuating SWR  (Read 3432 times)
KG4PLA
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Posts: 5




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« on: March 20, 2014, 01:31:55 PM »

Hey Folks,

I did my best to look thru the posts and find solutions before I addressed the Antenna gurus.

Setup:

Icom 706 MKII  grounded three feet away from grounding rod 6 foot down.
MFJ 904H tuner grounded same manner as rig.
RG213 coax..new and not damaged.

Hy-gain av14avq vertical, ground mounted...no radials and grounded to grounding rod 8 foot down and only 6 inches away from the ground point on the antenna. It is about 6 foot from a tree but not touching any branches at all as I have trimmed it.

It stands about 30 feet from my house. I painted it camo due to restrictions.

I can tune 40m band (CW op btw) with no problem. The SWR is flat and steady. I can tune the 20m band also flat but intermittently the SWR will spike to about 3:1 and I don't like it!

I checked the ground, the PL259s, and the clamps are all tight. What are your thoughts?

It wouldn't be the worst thing to be stuck on 40m but ..well..you know.

Thanks and 73

Dave
KG4PLA








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AA4PB
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« Reply #1 on: March 20, 2014, 02:14:33 PM »

A 14AVQ ground mounted should have radials. Try laying out a few on top of the ground and see if that doesn't help.
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KB4QAA
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Posts: 2450




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« Reply #2 on: March 20, 2014, 02:33:04 PM »

Lost my post, the site is slow and buggy.

Most likely RF into the radio / Common mode current on the feedline, or ground loop in your less than optimal grounding scheme.

1.  Add some radials per the manual.   
Can it be used without?  Sure.  And a rusty coat hanger can be use for an HF antenna too!  Smiley   As a novice I proved the 18AVT without radials performs worse than a random wire worked against ground.

-If nothing else, put 8 radials, roughly equal in length to height of antenna, length not critical.  Anything 28AWG or larger, insulated/uninsulated, solid/stranded.  Just do it. 

This should provide a good stable impedance, and I guarantee will blow your socks off, performing at least 3 times better than your present situation.

Others:
-Disconnect the radio and tuner grounds. You have a loop at present.  They aren't necessary anyhow.
-Add a 1:1 current UNUN/Isolator in the feedline close to the antenna.

That is a start, and I would be surprised if this doesn't solve your problems.  bill
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KG4PLA
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Posts: 5




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« Reply #3 on: March 20, 2014, 02:48:40 PM »

AA4PB andKB4QAA
Thanks for the quick reply.
Looks like the common idea here is to add some radials...will try that.
Also, will try disconnecting the ground to the rig and tuner.
I appreciate your input and I will post the results when I give those ideas a go.
Presently getting 579 reports and just figured it was because the vertical is in a less than optimum spot.
Thanks agn,
db
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W8JI
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« Reply #4 on: March 21, 2014, 05:30:16 AM »

Ground loops will not case the problem you are having, but lack of an adequate antenna ground sure can.

National fire and safety codes require any antenna cables entering the house to be grounded to an entrance ground that is bonded in common with the mains entrance ground. NEVER have a shack ground that floats independent of the mains ground. You can actually be better off without that shack ground than having one that is independent.

If you do NOT have a proper entrance ground, you should always disconnect the coax and move it back out to the antenna if bad weather threatens. Don't have it near the house or people!

I wonder if you have paint across trap insulation?

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WX7G
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« Reply #5 on: March 21, 2014, 07:59:15 AM »

It sounds like a loose connection. While monitoring the VSWR with low RF power a wooden stick can be used to push/pull on various parts of the coax, ground connection and the antenna itself.
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N4JTE
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« Reply #6 on: March 21, 2014, 02:43:10 PM »

Lack of radials on any antenna that needs them will lead to many problems, including the one you describe.
Bob
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KG4PLA
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Posts: 5




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« Reply #7 on: March 21, 2014, 07:41:08 PM »

Hey Folks

Update.

OK, so I followed the recommendations here and also went to the manufacturers webpage and followed their instructions to the letter for radials.
I am somewhat space limited and therefore one radial had to be partially draped over a brick retaining wall...only the last 10 inches or so.
I placed 8 radials down and cut them the size published in the manual.
I didn't bury them but laid them straight on top of the ground.
The 40m band is flat as before.
The 20m band went sky high on the SWR and even if I adjusted the length of the 20m portion of the antenna it was not tunable.
10 and 15m, which I rarely use, was pretty flat.

I thought, if the 20m band was ok but fluctuating before I put radials down then maybe I could remove the 20m radials and see what happens...this is what I did and the band is flat now...and stable.

I have had this antenna for a few years and never had radials installed before and it was ok...maybe Georgia dirt sucks!

Anyway, the 6 radials now seem to have solved the issue and I thank you for all the input.

Good night,

Dave

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W8JI
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« Reply #8 on: March 22, 2014, 04:58:35 AM »

Antenna manuals are almost never good.

If use only a few radials that are not in solid earth contact, and if you happen to make the radials resonant as a 1/2 wave on some band, that band will be useless.

The fix for this is not to remove radials, but to interlace radials that are about .2 wavelength long.

For example, if I laid a radial system out on ground surface with a few radials 20 feet long or so, they would probably make a very poor ground on 20 meters.  The velocity factor caused by the soil would make the radials look around 1/2 wave long, so each radial might have hundreds of ohms at the vertical.

If I interlaced some 10 foot radials, each of those radials might look like 50-100 ohms on 20 meters. The interlaced radials would act like a good ground.

Manufacturers either don't understand the complexity, or they avoid telling people things that complicate the installation. They would rather customers have a weaker signal than scare them.
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AA4PB
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« Reply #9 on: March 22, 2014, 06:43:25 AM »

When laying on the ground, the radials will NOT be resonant and it won't be necessary to cut them to specific lengths. More short radials are better than just a few long radials. Ideally make them as long as the antenna is high provided that they are all laying flat on the ground.

It's when radials are suspended in the air that they need to be cut to 1/2 wavelength long in order to provide a low impedance at the feed point.
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WX7G
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« Reply #10 on: March 22, 2014, 06:54:16 AM »

I know you meant to say 1/4 wavelength.
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W8JI
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« Reply #11 on: March 22, 2014, 09:11:07 AM »

When laying on the ground, the radials will NOT be resonant and it won't be necessary to cut them to specific lengths. More short radials are better than just a few long radials. Ideally make them as long as the antenna is high provided that they are all laying flat on the ground.

It's when radials are suspended in the air that they need to be cut to 1/2 wavelength long in order to provide a low impedance at the feed point.


That is not true. It is a myth.

Even when buried, radials exhibit resonances.  I made a series of measurements here on 40 meters that showed strong standing waves on bare aluminum radials about 2inches deep.

When the wires are laying on the ground, the effect can be very strong.

Of course every soil is different,  but many people have observed this effect. N6LF for another.
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KG4PLA
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Posts: 5




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« Reply #12 on: April 03, 2014, 08:08:48 PM »

OK
So, really haven't changed much since the last post.
My SWR is stable now on all bands.
I have stepped it down a bit on power to 70 W but I can't see how that would make a diff.
Could it have been that the rig or tuner just needed a nit of work after sitting for a couple of years in a box?
db
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KH2G
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Posts: 333




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« Reply #13 on: April 04, 2014, 03:38:38 PM »

Any piece of electronics that has been sitting for years needs to be checked before using and type of storage can make a huge difference.
regards,
Dick.
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