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Mobile Antenna Question
- Thread starter psycho
- Start date
it's all about the bonding, get a LOT of goood copper bonding straps, and copper foil.
As long as the body is conductive it does not matter what the material is. It could be steel or aluminum, brass, or any other metal and nobody will be able to tell the difference. You would be surprised how well an aluminum roof on an RV works.
This is absolutely true. There are people that make a big deal about the material, saying things like metal x is less conductive than metal y, but the difference in conductivity is so slight that you will never notice the difference. Further, only a very small part of the RF actually travels inside the metal reducing the differences even more...
The DB
On an aluminum Kenworth W900 ohms from rear of cab to bottom of door frame dropped A huge amount.
On all my all steel Volvo 770's ohms from back of cab to bottom of door frame were zero ohms.
On My steel Chevy Tahoe ohms from roof to frame are zero.
On my aluminum Peterbilt 379 ohms from mirror bracket to bottom of door frame dropped by 5 ohms even with braided ground strap.
needless to say the Volvos talked 60 miles mobile to base
the Kenworth was lucky to talk 30 miles mobile to base
the Peterbilt was lucky to talk 30 miles mobile to base
the Chevy talks 95 miles mobile to the same base
all as predicted
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If the aluminum was a continuous sheet there would be no difference. Most aluminum panels are actually glued and the problem is with bonding and not with the material itself. The other issue is with the binding of dissimilar metals which leads to high resistances in places. Just be glad you don't have a fiberglass cab.
I loved my fiberglass Peterbilt 387 with stacked dipoles. It talked 40 miles mobile to base and probably would have talked farther using same radio that was used in those other trucks.
Stacked dipoles? Try getting a decent match and performance out of a regular antenna that requires a groundplane. Try a magmount too. 
So you are saying your steel Chevy, which should be among the worst if metal made a difference, is actually doing the best? Thanks for proving my point for me.
The one I would have guessed that would do the worst among the semi's would be the Peterbuilt as a mirror is about the worst possible location for an antenna. The Kenworth and the Volvo should be about the same, and means how they aren't, that resistance reading you got on the Kenworth is more than likely the cause.
Something else that is affecting the semi's antennas could be as simple as the design of the vehicles in question. You have already shown that they are not equal with resistance measurement readings, all three have different readings. Different vehicles have different circumstance, and by extension different results.
It also looks like two of those vehicles need some bonding work done. All vehicles should have a resistance of 0 ohms between the antenna's mount and the frame. If they don't something in there is working against your antenna, and it could be as simple as the grease in the door hinges or a layer of paint between two pieces of metal...
Let me put in perspective how resistive these metals you are claiming you are seeing a difference is. The resistance of stainless steel is 0.00000069 ohms per meter. To all but the most sensitive of measuring equipment (and really even including this equipment) that is 0 ohms, and to get a reading of even 1 ohm of resistance you would need significantly (and by significantly I mean several orders of magnitude) more metal in between than any car/truck/semi on earth has, well more than a million times in fact (that is an understatement if ever there was one), and that is the most resistive metal used for antennas and ground planes and such, it has so little resistance that it is treated as no resistance. Anything with less resistance on a vehicle will make so little difference that you will never be able to measure/detect it, even if you had a theoretical perfect conductor that doesn't actually exist in reality for comparison.
The DB
I tried it without any luck on the 387. Braided ground straps for days. Eliminating all the ground straps and building the phased dipole system solved all my problems on that particular truck. I wish I had a chance to take my current station and put it back in that truck just to see how it would work nowadays.Stacked dipoles? Try getting a decent match and performance out of a regular antenna that requires a groundplane. Try a magmount too.
the Volvo had an unfair advantage as I had stacked elements mounted to the rear of the vehicle one on each side grounded directly to the steel bracket which was bolted directly to the steel body. I gave up on the Kenworth and resorted to a conventional single antenna on the mirror bracket mounted lower as it was my best ground at the time without major modifications.So you are saying your steel Chevy, which should be among the worst if metal made a difference, is actually doing the best? Thanks for proving my point for me.
The one I would have guessed that would do the worst among the semi's would be the Peterbuilt as a mirror is about the worst possible location for an antenna. The Kenworth and the Volvo should be about the same, and means how they aren't, that resistance reading you got on the Kenworth is more than likely the cause.
Something else that is affecting the semi's antennas could be as simple as the design of the vehicles in question. You have already shown that they are not equal with resistance measurement readings, all three have different readings. Different vehicles have different circumstance, and by extension different results.
It also looks like two of those vehicles need some bonding work done. All vehicles should have a resistance of 0 ohms between the antenna's mount and the frame. If they don't something in there is working against your antenna, and it could be as simple as the grease in the door hinges or a layer of paint between two pieces of metal...
Let me put in perspective how resistive these metals you are claiming you are seeing a difference is. The resistance of stainless steel is 0.00000069 ohms per meter. To all but the most sensitive of measuring equipment (and really even including this equipment) that is 0 ohms, and to get a reading of even 1 ohm of resistance you would need significantly (and by significantly I mean several orders of magnitude) more metal in between than any car/truck/semi on earth has, well more than a million times in fact (that is an understatement if ever there was one), and that is the most resistive metal used for antennas and ground planes and such, it has so little resistance that it is treated as no resistance. Anything with less resistance on a vehicle will make so little difference that you will never be able to measure/detect it, even if you had a theoretical perfect conductor that doesn't actually exist in reality for comparison.
The DB
I'll just leave this thought, especially for rabbiporkchop.
If aluminium is so bad for RF conductivity why are most antennas made from aluminium tubing?
If aluminium is a bad conductor then why are most overhead high voltage cables made from it?
Answer:
Electrical conductivity of steel, 6 million Siemens per metre, electrical conductivity of aluminium, 38 million Siemens per metre.
If aluminium is so bad for RF conductivity why are most antennas made from aluminium tubing?
If aluminium is a bad conductor then why are most overhead high voltage cables made from it?
Answer:
Electrical conductivity of steel, 6 million Siemens per metre, electrical conductivity of aluminium, 38 million Siemens per metre.
I agree if the body of a truck was stamped out of a single piece of aluminum the body would probably be more receptive to providing a decent ground plane.In
regards to mobile antennas though when running a radio with an extremely low noise floor like -125db using copper in my antenna eliminated some noise that covered up some weak signals that were not heard with an aluminum antenna on the same vehicle.A big base antenna made of aluminum with a radio with receiver sensitivity of .3 uv would have a difficult time hearing the same weak signal I could hear in my truck sitting under the base antenna with my copper antenna and .125 uv noise floor. If I was running an aluminum predator 10k, it would be about equal between the 2. Aluminum has it's place in different situations. I prefer less noise for qrp situations
though.
regards to mobile antennas though when running a radio with an extremely low noise floor like -125db using copper in my antenna eliminated some noise that covered up some weak signals that were not heard with an aluminum antenna on the same vehicle.A big base antenna made of aluminum with a radio with receiver sensitivity of .3 uv would have a difficult time hearing the same weak signal I could hear in my truck sitting under the base antenna with my copper antenna and .125 uv noise floor. If I was running an aluminum predator 10k, it would be about equal between the 2. Aluminum has it's place in different situations. I prefer less noise for qrp situations
though.
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