Is there a away to make a 6 meter covert mobile antenna. I am looking for a way to make an antenna that will work on my FD VHF-LOW freq of 46.18MHz. I have seen under bumper antennas for the 2 meter band. I'm assuming that there is a reason that these are not made for the freq I want, but maybe it's because of lack of use in this band anymore? Ideas?
6 meter mobile covert?
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mmckenna
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The "behind the bumper" antennas are extremely poor performers, just like the "license plate" antennas. They are only designed for short range operation where stealth is needed. Not a good solution for much of anything On 46MHz, your antenna is going to be close to 5 feet long for a 1/4 wave. Once you start getting shorter than that, coils are required, nothing wrong with that, they work find on base loaded CB antennas, but they start performing poorly the shorter you get. Trying to hide a low band antenna is going to be difficult if you want any sort of performance. I've never seen any "stealth" low band antennas. Just about everyone around here uses the full 1/4 wave whip. The town I live in has their public works on low band, and they are using base loaded whips. Even with that, they are 4 feet long.
Some home brew/experimentation on your behalf would be necessary, as anything stealth would need to be designed around the vehicle. I'm sure there are some novel ways to make something work, just not sure how much performance you are willing to trade for stealth.
Some home brew/experimentation on your behalf would be necessary, as anything stealth would need to be designed around the vehicle. I'm sure there are some novel ways to make something work, just not sure how much performance you are willing to trade for stealth.
n5ims
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Covert antennas for the VHF-Low band are quite rare (if you can find any at all) simply due to physics. Their 1/4 wave length is quite long vs those 2 meter antennas you mention. Think 19" for 2 meters vs about 5 feet for your VHF-Low solution. Shrink both the same percentages and you'll still end up with something not really "stealthy" on VHF-Low.
Probably the closest thing to stealth you'll find is some modification of those old AM/FM/CB antennas, and even those will stand out since the current car antenna design no longer looks anywhere near those old thick metal ones from auto's past. One other stealthy route may be to get one of those loaded 1/4 wave VHF-Low antennas that appear to be a standard CB antenna. Put it on the right vehicle and it'll look like somebody and his CB.
Probably the closest thing to stealth you'll find is some modification of those old AM/FM/CB antennas, and even those will stand out since the current car antenna design no longer looks anywhere near those old thick metal ones from auto's past. One other stealthy route may be to get one of those loaded 1/4 wave VHF-Low antennas that appear to be a standard CB antenna. Put it on the right vehicle and it'll look like somebody and his CB.
Robinsmark
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k9rzz
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I agree with all of the above. In my limited with experience with shorty CB antennas, this design worked very well and I think it's because the loading coil is up above the base. Way back when, I remember someone saying; "Never ever bumper mount a base loaded mobile whip." and those words have always stuck. Can you buy one like this for 47mhz? No, probably not. Perhaps you could modify an existing one to work, or simply run it through a tuner. If you want it to work on a 4mhz spread, you'll need a tuner anyway, unless you're just listening to one or the other.
I was always amazed how well this design worked. I had it on the middle of my roof and if I could hear them, I could work them (locals). YMMV
I was always amazed how well this design worked. I had it on the middle of my roof and if I could hear them, I could work them (locals). YMMV
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jhooten
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The laws of physics say that as the physical length of the antenna approaches 1/16 wave it also approaches the performance of a dummy load.
Dummy loads are simple 50 ohm resistance with nearly all of the transmitter output being converted to heat. An antenna is an inductive resonate device that in simple terms makes the energy leave the antenna and radiate out in a given pattern. Remember that for a 1/4 wave antenna the vehicle itself is half of the system. A 1/2 wave antenna doen't need a ground plane, but getting longer rather defeats the purpose of the exercise.
It's all about having the right length radiator attached to a ground plane, with the system resonating at the transmitter frequency. Physically shortened antennas have an inductive coil to make the entenna electrically longer and resonate, and keep the reflected energy from nuking the PA. By the time you have enough of a coil to make a very short antenna resonate at the desired (and very narrow) frequency it's probably going to make more heat than radiate signal, and have a very low power rating. There is a big difference between 5 watt CB's and 60 watt VHF land mobile radios.
Receive is easier: our old 39.5 portable bricks would receive fair around town (brute force within 2 miles of a 100W base station) but had no range unless you pulled the whip out. VHF-hi and UHF 'covert' antennas are available and work OK in brute force environments simply because the wavelength is shorter, but their range and performance is notoriously awful.
None of my commercial suppliers have anything in the 25-50 Mhz range. That's not saying they aren't out there; it is theoretically possible to couple to the vehicle itself, but polarity will be wacked, performance will still be marginal and cost high. I don't even want to think about EM exposure levels. Best of luck.
Dummy loads are simple 50 ohm resistance with nearly all of the transmitter output being converted to heat. An antenna is an inductive resonate device that in simple terms makes the energy leave the antenna and radiate out in a given pattern. Remember that for a 1/4 wave antenna the vehicle itself is half of the system. A 1/2 wave antenna doen't need a ground plane, but getting longer rather defeats the purpose of the exercise.
It's all about having the right length radiator attached to a ground plane, with the system resonating at the transmitter frequency. Physically shortened antennas have an inductive coil to make the entenna electrically longer and resonate, and keep the reflected energy from nuking the PA. By the time you have enough of a coil to make a very short antenna resonate at the desired (and very narrow) frequency it's probably going to make more heat than radiate signal, and have a very low power rating. There is a big difference between 5 watt CB's and 60 watt VHF land mobile radios.
Receive is easier: our old 39.5 portable bricks would receive fair around town (brute force within 2 miles of a 100W base station) but had no range unless you pulled the whip out. VHF-hi and UHF 'covert' antennas are available and work OK in brute force environments simply because the wavelength is shorter, but their range and performance is notoriously awful.
None of my commercial suppliers have anything in the 25-50 Mhz range. That's not saying they aren't out there; it is theoretically possible to couple to the vehicle itself, but polarity will be wacked, performance will still be marginal and cost high. I don't even want to think about EM exposure levels. Best of luck.
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