wa8yxm
Wherever I happen to park
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Joined: 07/04/2006
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Since you have a metal roof.... http://www.ccrane.com/antennas/fm-antennas/fanfare-fm-antenna.aspx
Nothin adds excitment like something that is none of your business
John is Near Kenwood TS-2000 housed in a 2005 Damon Intruder 377
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Healeyman
Carrollton, TX
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Joined: 09/08/2006
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bananadanna wrote:What are my options?
Sirius & XM
Tim
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bananadanna
Cambridge, MA
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Joined: 11/30/2005
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I called the Crane tech people and they think I can use this antenna horizontally. I am thinking of trying it. It comes with RG59 cable (70 ohm).
I did see that RG8X cable (50 ohm) is widely used and perhaps a better match to the radio impedance. But further reading said it was not a huge factor and 50 or 70 ohm would both be ok.
Dan
02 Freightliner Sprinter 2500 long tall home brew conversion
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wa8yxm
Wherever I happen to park
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I'd go with the cable they suggest (RG-59 or RG-8x either one)
Once I knew you had a metal roof, options went away fast. I searched for a rooftop antenna but alas, I did not find it. That one looks to be just what you are looking for. Glad my Google skills were of assistance
just remember GOOGLE IS your friend.. Search terms "FM ANTENNA"
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sbingham
Payson, AZ
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Joined: 10/01/2004
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You need a 1/4 wave vertical antenna. Your metal vehicle/roof is your ground. Amplifiers are a waste of money. I would guess your feedline (cable) needs to be 75 ohm. Use a standard car radio replacement cable. A half wave for the center of the FM band is around 5' (60"). Use a standard car antenna and mount it on the roof. Simple and effective. If reception is still a problem, replace your current radio with any Pioneer Super Tuner model (the tuner section is the same from the cheapest to the top of their line). They have excellent sensitivity and selectivity - in fact superior to my VERY expensive Onkyo tuner in my hi-fi system.
Oh yeah, I own an FM station (KRIM-FM) so I am pretty familiar with what works - although I am not an engineer (but a ham for 25 years).
Steve and Bobbie (and Shotgun - my 115 lb dog)
Down sized to a Nash 25S 
My web page is www.dustylens.com
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bananadanna
Cambridge, MA
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Thanks guys, you've been a great help.
I sure thought you could find out about anything with Google. The learning curve for antennas is pretty steep. And my troubles finding out about impedance matching were pretty bad.
My van is 9'6" with its roofrack and the combination of offroad camping and city street trees was what lead me to look for a horizontal solution. My nerd hairs have gotten raised and now I want to try some experiments. The huge roofrack makes a 60" crosswise or 120" lengthwise horizontal antenna easier for me than almost anyone else.
I think the comparisons will be between a vertical 1/4 wave whip, the Crane 58" (monopole?) mounted horizontal per their tech support, a folded-dipole home antenna with a 300/75 balun, and a horizontal 3/4 wave DIY J-pole. It'll take experiments to see if reflections from the metal roof are a problem. I did see a yagi calculation to use it as a reflector but the distance could be too large for my use (around 18" IIRC.)
I am hoping directionality won't mean I need two orthagonal horizontals...
If vertical reception is better than good horizontal reception then I will try to swivel mount the antenna for use in camp. (See my other mentions of wifi/cell mast.) The ground plane 1/4 wave whip would obviously need vertical-only.
If the horizontals all fail I may retreat to a standard $30 retractible 1/4 wave whip.
I do in fact have a Pioneer Premier Supertuner. I was astonished at its inability to get enough signal. I get many college stations in Cambridge and they disappear much in just a few miles. The big watt NPR station only gets about 25 mi of good reception. The Subaru with a factory radio will get around twice that. The stubby standard Sprinter antenna is not well regarded to say the least.
Can someone explain the use of a choke? I saw mention of using four turns of the lead in cable to accomplish this.
And a background article on horizontal vs vertical antenna
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tuxsteveo
Wisconsin
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Joined: 07/03/2005
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When I got my used TT the FM antenna on the roof was very bad I unscrewed it and used the old coax to fish in a new piece of rg58 and mounted a two meter/ 440 ham antenna in it's place about 30 inches tall. I put a bnc on the end and use an adapter to go to the FM radio. When I need to use it as a base I just hook up a ham radio and use it. It works great on FM, NOAA weather, and ham bands. No horizontal solution will work as good one sticking up 30 inches.
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sbingham
Payson, AZ
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"If the horizontals all fail I may retreat to a standard $30 retractable 1/4 wave whip." actually, this where you should START!
95% of ALL transmit/receive problems relate to the connectors or feed line!
Make sure your cable does not have a kink. Any kink will cause an impedance bump that can easily drop your receive signal 20-50 dB. A bad connector can be even worse.
Over semi flat terrain, any 100K station should easily reach 100 miles. Easily. The inability of your Super Tuner to receive has nothing to do with the receiver - and everything to do with your antenna, feedline, and connectors.
Keep in mind that almost all commercial FM stations use circular polarization. This means, in theory, that there should be no difference in vertical verses horizontal polarization. The trouble with horizontal polarization is that you need at least a 1/2 wave clearance - and preferably 1 wave length free space between the horizontal element and the ground - your roof. Hence, this would be a VERY poor choice in your instance.
Again, this what I do for a living.
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bananadanna
Cambridge, MA
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sbingham wrote:"If the horizontals all fail I may retreat to a standard $30 retractable 1/4 wave whip." actually, this where you should START!
95% of ALL transmit/receive problems relate to the connectors or feed line!
Make sure your cable does not have a kink. Any kink will cause an impedance bump that can easily drop your receive signal 20-50 dB. A bad connector can be even worse.
Over semi flat terrain, any 100K station should easily reach 100 miles. Easily. The inability of your Super Tuner to receive has nothing to do with the receiver - and everything to do with your antenna, feedline, and connectors.
Keep in mind that almost all commercial FM stations use circular polarization. This means, in theory, that there should be no difference in vertical verses horizontal polarization. The trouble with horizontal polarization is that you need at least a 1/2 wave clearance - and preferably 1 wave length free space between the horizontal element and the ground - your roof. Hence, this would be a VERY poor choice in your instance.
Again, this what I do for a living.
Okay, thanks for the insight. I had worried about being unable to get a horizontal antenna very far above the metal roof. I think the tech guy at Crane may not have given me the correct answer to that very question. I'll track down a retractible vertical antenna to use.
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