| Author |
Message |
    
Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 171
Registered: 9-2004
| | Posted on Monday, April 02, 2007 - 8:29 am: | 
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Heres one that I was thinking about the other day.Does anyone have any thoughts on which has the greater efficiency,Magnetic coupling such as in a mag mount antenna or a directly grounded antenna such as a whip or similar antenna which is directly grounded to the car body.I often wonder how efficient magnetic coupling really is.My thoughts are that directly grounded antennas should be a better scheme as they are a better match because they are directly and electrically connected to the system
Any comments would be greatly appreciated.
Tnx Bob CEF451
Robert L. Spicer
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Tech237
Moderator
Username: Tech237
Post Number: 734
Registered: 4-2004
| | Posted on Monday, April 02, 2007 - 8:48 am: |
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Directly grounded will beat a mag mount all else being equal.
Tests we did in Australia several years back SEEM to support this too. I say seem as we were never able to run both antenna's from the same source while they were located at exactly the same spot at exactly the same time. Within these limitations the directly grounded antenna did show a higher field strength at 1, 2 and 5 mile ranges we tested. Both antennas were 5/8th wave on 2m.
Simon
Tech237
KD7IEB
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Tech833
Moderator
Username: Tech833
Post Number: 1639
Registered: 8-2002
| | Posted on Monday, April 02, 2007 - 10:17 am: |
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Tests done at VHF and UHF frequencies show a dramatic improvement by directly grounding the antenna. Since the lower you go (in frequency), the more dependent the antenna system is on grounding, I would guess that with CB frequencies, the direct ground would be more essential.
Of course, a magnet mount is much better than nothing. |
Romstar
Member
Username: Romstar
Post Number: 61
Registered: 3-2007
| | Posted on Monday, April 02, 2007 - 8:48 pm: |
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The only way to improve a magnetic mount antenna is to sand the pain away from the surface of the vehicle, and couple the base of the antenna to the vehicle with a conductive paste.
This of course makes a total mess out of the paint, but does avoid drilling a hole.
It is *NOT* as good as a direct ground, but it does improv the grounding of a magnet mount.
Sort of a cross between the two ideas.
When you sell the vehicle, just sand and use a can of touch up paint. Unless you are looking for perfection.
All things being equal, if you can physically ground the antenna, you should.
Romstar |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 3585
Registered: 7-2004
| | Posted on Monday, April 02, 2007 - 11:29 pm: |
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i have a 'trailer hitch receiver' mount we made for...anyway, it also has a ball mount welded to a plate on it. it then has 2 pieces of #6 copper bolted to the 'mount' on one end & the frame on the other. with an mfj 802 field strength meter turned down to 'lowest' sensitivity, my grant xl at approx 2 watts carrier & predator 10k antenna, there was NO noticeable difference if i put it back on its triple mag mount. no matter where we turned the sensitivity, to get a reading of any #-20-50-80-100-whatever, it didn't change noticeably on the mag or on the 'pole'.
would it change with a hole drilled through the roof? doesn't the 2 copper wires count as a 'direct ground'? the antenna is still 'above' the sheet metal-in 'use', when the antenna is on the magnets in the center of the roof, it is LESS directive to stations hearing me then when it is on the 'pole', or even on the mag 2' forward or back of where i drive with it. based on the FS meter readings & 'radio checks' with the pole & magnets, i never drilled a hole. |
Tech237
Moderator
Username: Tech237
Post Number: 736
Registered: 4-2004
| | Posted on Tuesday, April 03, 2007 - 9:33 am: |
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Pat at what distance did you take those measurements?? If you were too close (less than a few wavelenghts away) you would be reading the near field which may not change as much due to being close to the source. THis is the reason why, in the tests we ran, we used ranges of 1,2 and 5 miles away from the source. It removes any possibility of overload on the FSM.
Simon
Tech237
KD7IEB
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Tech833
Moderator
Username: Tech833
Post Number: 1641
Registered: 8-2002
| | Posted on Tuesday, April 03, 2007 - 2:31 pm: |
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A hole in the roof antenna will blow away a bumper or hitch mount antenna any day. So, a mag mount on the roof performing the 'same' as one on the hitch??? Think abou tit!! How much better would that hitch antenna work if it was in the middle of the roof!!
You are comparing apples and grapes. |
