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Author Topic: High SWR  (Read 598 times)
WD4HLD
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Posts: 4




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« on: July 24, 2006, 04:10:35 AM »

I just installed a Icom 706 and Diamond HV7A in my Jeep Liberty. The SWR is extremely high on 6 & 10 meters. I tried adjusting but it doesn't make any difference. The antenna is attached via a lip mount on the back door.

I know grounding is important but I thought since the mount is attached to metal, grounding would not be a problem.

The coax and PL259 are from the factory. I did not make them up.

Any help will be greatly appreciated.
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K0BG
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« Reply #1 on: July 24, 2006, 05:15:45 AM »

The term "Grounding" is often misused, and this is another case. Physically grounding an antenna mount to the chassis doesn't magically transform the mount into a ground PLANE. Remember, a vertical antenna's radiating element represents just one half of the antenna SYSTEM. The other half (in this case) is the body of the vehicle.

The high SWR is caused by the antenna being other than 50 ohms impedance (or close to it). This can be caused by excessive ground loss (including stray capacitances), or grossly out of resonance, or the antenna coil losses are high (or a combination of all of these).

If you use an antenna analyzer (MFJ 259B or similar), you can interpolate all of the above factors. My guess? It's grossly out of resonance.

Alan, KØBG
www.k0bg.com
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K4KWH
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Posts: 51




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« Reply #2 on: July 24, 2006, 09:32:38 AM »

The reason for the high SWR might be simply that, if the antenna is attached to that hinged door(?), it's not making good connections for RF due to oil and other factors. Hinges don't make good grounds. Try connecting a small (short) jumper to the
main body of the car and see if it makes a difference.

73

Jerry
K4KWH
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K0BG
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« Reply #3 on: July 24, 2006, 12:01:44 PM »

Apparently, I didn't make it clear. DC grounds WILL NOT magically make an antenna work like it had radials under it. This includes, but is not limited to, any kind a strap to any part of the body. What's more, RF just doesn't care if there is grease, oil, or dirt on the hinges.

Look at it this way. If you have a base station vertical, mounted on the ground with no radials around it, would you expect it to work well? Obviously not. And adding a ground rod isn't going to make it work any better. The same goes for a mobile vertical, only more so.

A vehicle is an inadequate ground plane for any frequency under about 60 MHz. It acts more like a capacitance to ground of between .002 uF and .004 uF depending on the frequency, the vehicle, and how well it is bonded and/or welded together, and where and how the antenna is mounted. A hatch is NOT a good ground plane. Adding a ground strap doesn't do any more for it than the aforementioned ground rod does for a base station vertical.

Alan, KØBG
www.k0bg.com
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W3LK
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« Reply #4 on: July 25, 2006, 07:34:12 AM »

Alan:

Reminds me of a line from Peter, Paul and Mary:

"...when will they ever learn..." Smiley

73,

Lon - W3LK
Baltimore, Maryland
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
AB2MH
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« Reply #5 on: July 25, 2006, 11:23:46 AM »

Read the reviews:

http://www.eham.net/reviews/detail/1563

What kind of SWR are you getting exactly?

From what I see, 1.6-1.7 is good for this antenna.
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WW5AA
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Posts: 2086




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« Reply #6 on: July 27, 2006, 07:24:25 AM »

Alan,

You must have missed all the posts about slamming a 1/4 wave vertical into the the ground and working every one you hear!  (snicker snicker)

73, de Lindy
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W3LK
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« Reply #7 on: July 27, 2006, 01:08:04 PM »

Alan:

If you haven't already, be sure to read the article in the July CQ about doing HF installs. Also the article on mobile antennas.

The install article has some interresting comments on antenna grounding and the antenna article shows a gentleman running a HamStick (r) and an AH-4 coupler with about five feet of wire between the two. Smiley

73,

Lon - W3LK
Baltimore, Maryland
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KE3HO
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Posts: 235




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« Reply #8 on: August 04, 2006, 07:50:29 PM »

<< HamStick (r) and an AH-4 coupler with about five feet of wire between the two. >>

Well, at least he has about five feet of wire that might act like an antenna, but what is the hamstick (r) for?  :-)

73 - Jim
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KF6NTE
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Posts: 8




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« Reply #9 on: December 18, 2008, 05:05:19 PM »

I have the same setup as you describe and have found that bonding the frame of the car has improved the SWR somewhat over what I used to see. But even then this antenna has very narrow resonance bandwidth in some cases even less than 30 or 40Khz. Having said that I get a very reasonable SWR bandwidth (below 2:1) in the entire 6m band. I can reach most repeaters with this setup. On 10 meters I have to tune it depending on the frequency swath I want to
work. Currently the FM band (29.5-29.7Mhz) can be
kept under 2:1 to a reasonable extent for about 50-75Khz. The 20m and 40m coils are a different story. especially the 40m. On 20m, I tuned to 14.185Mhz +/-
25Khz. I know anything outside this range will be warming up the air rather than radiating anything out
with any kind of efficiency.
Same story on 40m. Here I barely have any elbow room before the SWR goes to never-never land. You would be ill advised not to use an impedance coupler between your radio and this antenna (for protection) even if the matching efficiency is poor outside the range you can achieve at a time by clipping the tuning stubs. All that being said, If you want better efficiency you need to reduce coil loss ideally by increasing the reactance to resistance ratio. This is done by increasing its size (Hi-Q has antennas with monster coils that do just this). Yes, Mobile HF is an exercise in compromise.
--Luis NY6U
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