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Message |
    
Rick330man
Junior Member
Username: Rick330man
Post Number: 10
Registered: 9-2005
| | Posted on Thursday, November 17, 2005 - 1:44 am: | 
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I've tried a bunch of different antennas to try to get a base station set up at an aluminum mobile home. The problem is that the SWRs are almost flat on channel 1 but over 3:1 on channel 40. I get the same results with different antennas - antennas that give me good SWRs elsewhere. The antennas are my Radio Shack crossbow (1/2 wave base station antenna), my Workman Saturn (Mini-base antenna designed for no ground plane surfaces) and my Shakespeare 5205 (marine CB antenna). Any suggestions for correcting this?
Thanks.
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Road_warrior
Advanced Member
Username: Road_warrior
Post Number: 927
Registered: 7-2004
| | Posted on Thursday, November 17, 2005 - 7:22 am: |
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How high are you mounting each of these antennas??? If antennas are to close to
metal objects the reflection will throw
off SWRS. |
Nobodyknows
Intermediate Member
Username: Nobodyknows
Post Number: 125
Registered: 4-2005
| | Posted on Friday, November 18, 2005 - 2:22 am: |
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I live in a mobile home also, The structure itself doesn't seem to affect the swr. I have a push-up pole U clamped to the side of the "MH" and an antron 99 on the pole. 1.1 on ch1 and 1.2 on ch40. I did have a CLR2(1/2 wave GP) instead of the antron and had a little trouble getting it lower than 1.5 across the band but that CLR was always fussy wherever I mounted it.
Dumb question here but... Did you try setting the swrs to get 1.1 on ch20? And yeah I have to ask, do you have anything close to the antenna now?
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Road_warrior
Advanced Member
Username: Road_warrior
Post Number: 938
Registered: 7-2004
| | Posted on Friday, November 18, 2005 - 9:20 am: |
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That's not a dumb question Nobodyknows.
He's either getting reflection or SWRS are
not set for the middle of the 11 meter band.
Or SWR meter is not accurate. Could be a few
other things also.
JIM/PA/CEF 375 |
Marconi
Intermediate Member
Username: Marconi
Post Number: 496
Registered: 11-2001
| | Posted on Monday, November 21, 2005 - 9:44 pm: |
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Rick we tend to think that an antenna that is electrically connected to a flat metal surface, like your MH should give us a very good ground plane. That may be true in some cases, but I have seen situations where instead, it seemed like the flat metal surface just sucked the fire right out of the antenna instead.
I cannot explain that very well, but if true and it was not something else at play around the antennas, then maybe the current to the ground plan was for some reason greater than the current to the antenna radiator. Of course different antennas respond differently to metal nearby.
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Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 922
Registered: 6-2004
| | Posted on Tuesday, November 22, 2005 - 7:35 am: |
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Mount a 102" whip on the roof and use the mobile home as a counterpoise! Otherwise you might need to get the antenna fairly high above it.
Metal objects near an antenna effect it's performance. The ideal situation would be that the antenna was centered over your home and at least a full wavelength (about 36') above it, but that's probably not possible. So get it as high as possible and make sure your home is well grounded and your antenna is well grounded too.
I always wonder why I never see antennas mounted on Airstream trailers. Those things are awesome ground planes and just begging for an antenna farm. |
Bruce
Senior Member
Username: Bruce
Post Number: 3244
Registered: 9-2003
| | Posted on Tuesday, November 22, 2005 - 9:49 am: |
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My hamshack is a 12 by 50 foot alum room added to the house. EVEN on 75 meters that roof works good as a ground ........ Hollowpoint is right.
Now some ham antennas like that 102 inch wip are MADE to be mounted on the ground ..... OR ..... on a GROUNDPLANE .....Like the roof.....
The wip ( like that 102 inch ) is only 1/2 the antenna HAM Antennas like the hygain AV-18 or the 4 band Hustler ARE WIPS and NEED that ground to work well at all....... |
Rick330man
Junior Member
Username: Rick330man
Post Number: 14
Registered: 9-2005
| | Posted on Monday, November 28, 2005 - 12:28 am: |
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Thanks for all of the suggestions.
I am in the lower Florida Keys. The primary portion of the mobile home actually has your standard 3 tab shingle roof. But the antenna is set up over a second, but attached, porch roof which is made of two layers of powder coated aluminum which sandwich a s sheet of styrofoam.
It can get very windy down here and deed restrictions don't allow guy wires, so I can't get too high up. And the restrictions in the community don't allow much height above the home anyway. The antenna sits now about 5 and 1/2 feet from its base to the aluminum roof. That's about 17 1/2 feet above the ground.
The coax is wrapped around the mast (RF choke). I also noticed that the SWRs fluctuate a lot if I happen to touch the radio while I'm transmitting to check them. The latter had me thinking that there is something about my unti that is going to require a better counterpoise.
The SWR meter is fine. I checked it against another one and got essentially the same readings.
Any other ideas?
Thanks again. |
Mike0228
Junior Member
Username: Mike0228
Post Number: 34
Registered: 6-2004
| | Posted on Monday, November 28, 2005 - 7:00 am: |
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Haven't heard of wrapping the coax around the mast to function as an RF choke. I believe you're defeating the purpose unless the mast is nonconductive (white PVC pipe). I'd get rid of that and wrap about 20' of coax around a 2 liter bottle then tape it up so the turns don't overlap. Tape to the mast above and below the choke so as to keep it AWAY from the mast a bit.
Mac |
Rick330man
Junior Member
Username: Rick330man
Post Number: 16
Registered: 9-2005
| | Posted on Monday, November 28, 2005 - 9:19 am: |
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The mast is fiberglass, and the SWRs go up across the band when I unwrap and simply run the coaxial cable straight. |
Dx431
Senior Member
Username: Dx431
Post Number: 1126
Registered: 2-2002
| | Posted on Monday, November 28, 2005 - 11:40 am: |
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Rick330man, I would not use a fiberglass mast. Menards and Lowes has metal ant masts at a reasonable price. Get one and reinstall your ant. And, get an 8' copper ground rod and some heavey gage wire while you're there. Pound the rod in the ground next to the mast and attach the wire to the mast and the rod. This should help.
Mike0228, read the instructions on the A99.Unless Solarcon has changed their instructions since I baught mine, the instructions tell you how to make an RF Choke by coiling the coax around the mast just below the feedpoint of the ant. Back in 80's when everyone around here was installing the antrons, we installed the ant per instrucions, this included the coiling of the coax around the mast.It helps reduce TVI.
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Mike0228
Junior Member
Username: Mike0228
Post Number: 35
Registered: 6-2004
| | Posted on Monday, November 28, 2005 - 7:09 pm: |
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Wrapping around the mast is something I would never do but since I'm not an antenna engineer I'll leave it to the techs here to answer. Always have made mine as I described for A-99 and Imax 2000. IMHO I don't want my choke touching a metallic mast.
Mac |
Dx431
Senior Member
Username: Dx431
Post Number: 1128
Registered: 2-2002
| | Posted on Wednesday, November 30, 2005 - 8:16 pm: |
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Your coax will not know the diferrance between a metal pole and plastic jug.
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Al_lafon
Intermediate Member
Username: Al_lafon
Post Number: 121
Registered: 3-2005
| | Posted on Saturday, December 10, 2005 - 2:14 am: |
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Hello did anyone check out the coax here.
Key remark ( I also noticed that the SWRs fluctuate a lot if I happen to touch the radio while I'm transmitting to check them. )sounds like a lot of RF in the shack. put a known good
50ohm load on your coax and make sure its good
move it around some near the ends. Also are we
talking stock or high power here.you may need a Balun to keep RF out of your radio room.It will be a 1 to 1 Balun just look it up on line. Do not Ground your antenna to the mast cut some pvc pipe to install at the top of your mast so the
antenna is not grounded but do ground your coax
as near the ground as you can 8' copper ground rod . you may also make a rf coke with the coax
5 to 8 turhs about 10" across a few feet from base antenna this may help in keeping the RF out
of your shack. Hope i have been some help to you.
73s Al / N3IZR
of your antenna. |
Rick330man
Junior Member
Username: Rick330man
Post Number: 19
Registered: 9-2005
| | Posted on Sunday, December 11, 2005 - 7:09 pm: |
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Coax is fine. I've actually used several different runs just to be sure. None showed signs of shorts.
Since this post started, I think I have been able to isolate the problem to one of two possible sources. Culprit one may be when a section of aluminum soffit was brushing up against a portion of stainless steel mast making metal to metal contact. Culprit number two appears to have been some 1" by 3" aluminum spacers I used below the antenna but within 2 inches of the coax run at one point.
After eliminating these two (and maintaining a respectable looking RF choke), my SWRs are running 1.3 of 1.7 across the CB band.
Thanks for all of the suggestions.
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Mike0228
Junior Member
Username: Mike0228
Post Number: 36
Registered: 6-2004
| | Posted on Saturday, December 24, 2005 - 9:01 am: |
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"Your coax will not know the diferrance between a metal pole and plastic jug."
It will when the center conductor migrates because the radius is too tight unless you're using RG-58 which I would never use. Perhaps you could get away with 8X but I still wouldn't make the turns that tight. |
Hotwire
Advanced Member
Username: Hotwire
Post Number: 770
Registered: 1-2005
| | Posted on Saturday, December 31, 2005 - 2:41 pm: |
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Never had this type of install before. Could it be that it wants to be a mobile installation since its a mobile home? I would be sure to ground antenna to mobile home and home to earth ground. Since it is kinda like a vehicle I bet you that a 102 inch stainless steel whip right on top would work awesome....grounded of course.I'm sure it does not matter the antenna type but I have dicovered that no 2 installs are the same unless they are infact the same object antenna ect ect. Just some suggestions and opinions from my twisted mind. |
Slugo4449
Member
Username: Slugo4449
Post Number: 67
Registered: 7-2004
| | Posted on Thursday, February 02, 2006 - 9:06 pm: |
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If the metal roof is floating on top of a wooden structure, why don't you try running a simple ground wire and ground the roof?
I would also ground the radio and the gear. Wire is cheap.
Marty |
Drifter_8291
Junior Member
Username: Drifter_8291
Post Number: 23
Registered: 10-2004
| | Posted on Monday, February 06, 2006 - 8:23 am: |
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Hi I live in a mobile home also with metal roof and siding. My antenna an A99 is mounted on the side of my mobile home about 4 above the roof. My first try with 18 foot mini foam coax my standing wave was on channel 1 a 2 and on channel 40 a 1.2 ...I added 18 more feet of the mini foam coax and my swr dropped to on channel 1 to 1 and on channel 40 to a 1 ....... as for my highest channel 27.965 my swr is 1.3 and on the other end 26.705 my swr is 1.2 ..... I get out good and all... I have been wanting to get a Top One (astroplane) antenna but have been afraid to do so because I don't know how it would react to my mobile home and there is really no where else to mount the antenna....any ideas or anyone else run a (astroplane) Top One on a mobile home and what were your results? |
852
New member
Username: 852
Post Number: 2
Registered: 3-2006
| | Posted on Friday, March 03, 2006 - 3:00 pm: |
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hmmm, a 102' whip in the center of a mobile home.. now what kind of mount would you use for this, and would it really work?? |
852
New member
Username: 852
Post Number: 3
Registered: 3-2006
| | Posted on Friday, March 03, 2006 - 3:02 pm: |
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yeah drifter i was wondering the same thing, have you gotten the Top One astroplane yet? If So how does it perform? |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1007
Registered: 1-2005
| | Posted on Saturday, March 04, 2006 - 2:39 pm: |
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You would have to drill holes and make a bracket. |
Road_warrior
Senior Member
Username: Road_warrior
Post Number: 1263
Registered: 7-2004
| | Posted on Sunday, March 05, 2006 - 11:14 am: |
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I know a guy who mounted a 102" whip on
a truck wheel rim and set it on his house
roof. Worked good. Ran ground-wire from rim
to ground-rods.
852- Top One antennas perform good also. |
852
Junior Member
Username: 852
Post Number: 12
Registered: 3-2006
| | Posted on Sunday, March 05, 2006 - 11:34 am: |
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Thanks warrior, i'm pondering on a top one astroplane mounted on 20 foot mast in the yard to get away from the mobile home side.. I hear they perform well set up in this manner. |
