Being a new ham, I don't have a lot of experience and I was wondering if attaching an antenna directly to a mobile radio (no coax) would work or is the coax needed to get a good SWR? For example, take a Kenwood TM-V71A and attach a Comet CA-2X4SR. I was just thinking a setup like that might be good as a backpack rig or a good portable system.
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There are a couple of reasons why you'd want to use coax and why you wouldn't.
But first, CAN you attach the antenna directly? Sure, provided that the radio has a UHF connector on it. Otherwise, you'd need to use an adapter which would introduce some loss.
I'm certainly not an expert on these two items, but I would guess that it would be durned near impossible to just plug the antenna into the radio, unless you had a four foot (however long the antenna is) clearance between the radio and the nearest wall.
Overall, I think you'd be well advised to always use some coax in the mix. Why? Because of the huge possibility of straining the connector. Without the coax, one decent BUMP! could ruin your day. Coax adds flexibility.
Nice radio!
73
Luck, WA4STO
http://www.qrz.com/db/wa4sto (http://www.qrz.com/db/wa4sto)
Hello,
Most mobile antennas expect to see a counterpoise of some kind, which of course ends up being the car chassis in many cases. So, if you plug in a mobile antenna directly into the radio via an adapter, you will most likely need to connect a wire to the radio chassis and let it dangle down. I would start with a quarter wave wire, and experiment with length for best SWR.
Gil.
From undisclosed location in hostile in-law country...
The antenna you mention has a PL-259 (male HF) connector according to its literature. It would be advisable, simply in the interest of being nice to the radio, to put something like a short piece of coax between them so that youre big broadband whip isn't stressing the connector interface on the radio. That's just one reason.
Your idea of a packable "package" is a good start in terms of bulk (if you must have all bands the radio is capable of) but, wait.... did you also want to power that TM-V71 at its full power output? Now you start to add non-hump-friendly battery power.
Welcome to the forum.
:)
Back to the fray; people really should secure their internet capabilities you know...
8)
And I would suggest not operating with that antenna too close to your head...
Gil.
Quote from: gil on August 24, 2013, 07:53:21 PM
And I would suggest not operating with that antenna too close to your head...
Gil.
(back in the Badger country now, thankfully)
Gil mentions the other reason big-time & man-portable whips or other close proximity radiators at high frequencies can bite you, even at much lower power.
Several websites available to do the same thing, but amounts to an RF safety assessment.
Try this one and do an apples to apples comparison of 50w at both 7mHz and 440mHz, 3dBi gain for both, proximity 1 foot.
http://www.wirelessconnections.net/calcs/rfsafety.asp
While the RF exposure at VHF/UHF with an antenna from a 50 watt transmitter that close to ones body is a serious issue, the duty cycle is likely to be very low because drawing that much current is going to run any man portable sized battery dead in very short order making it sort of a moot point. In other words it's not a very feasible setup to begin with.
Quote from: gil on August 24, 2013, 03:07:24 PM
Hello,
Most mobile antennas expect to see a counterpoise of some kind, which of course ends up being the car chassis in many cases. So, if you plug in a mobile antenna directly into the radio via an adapter, you will most likely need to connect a wire to the radio chassis and let it dangle down. I would start with a quarter wave wire, and experiment with length for best SWR.
Gil.
Good point about the counterpoise. I actually added a counterpoise "rat tail" to my HT and that has helped it out a lot.
BTW, while thinking about this, I had another idea. How about using an umbrella as a base plane? Anybody seen anything like that tried?
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Quote from: KC9TNH on August 24, 2013, 07:16:11 PM
Your idea of a packable "package" is a good start in terms of bulk (if you must have all bands the radio is capable of) but, wait.... did you also want to power that TM-V71 at its full power output? Now you start to add non-hump-friendly battery power.
Yeah, didn't think about the power for a 50W transmitter. Now that I have a better understanding of the range on a 5W HT (which it's actually pretty good), I think an HT is just fine and will run a lot longer on the internal battery. I'm actually looking at picking up a few Puxing PX-UV973 HT radios. They get really good reviews and have a cross band repeater capability. I'm thinking that I might be able to put together a rig with one of them in a waterproof box, a slim Jim antenna, and a solar panel. Throw it up in a tree and have a low profile local repeater system.
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Quote from: gil on August 24, 2013, 07:53:21 PM
And I would suggest not operating with that antenna too close to your head...
Gil.
I did consider that, but hasn't gotten around to looking into what the exposure would be.
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Anyway, thanks everyone for the feedback.
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I've got a Yeasu FT-8900R fully backpack portable with a 10aH battery, a buddipole antenna, YoYo-Tenna, and Opek HVT-400B (what can I say, I like having options in my antennae) along with 250' of 550 cord. The whole thing weighs in at just under 30# and can be operated on the move.
I looked around a little and found someone who had a similar idea on the umbrella antenna:
http://www.digitalcartography.com/n0eq/umbrella.htm
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That's pretty slick!
QuoteHow about using an umbrella as a base plane? Anybody seen anything like that tried?
The best idea I've heard in a long time!
Gil.
Quote from: KC3AOL on August 24, 2013, 07:53:41 AM
Being a new ham, I don't have a lot of experience and I was wondering if attaching an antenna directly to a mobile radio (no coax) would work or is the coax needed to get a good SWR? For example, take a Kenwood TM-V71A and attach a Comet CA-2X4SR. I was just thinking a setup like that might be good as a backpack rig or a good portable system.
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The feedline is not what resonates the antenna but the outside of the coax shield can act as a counterpoise if there is a lack of a proper ground system. What resonates the antenna is actually adjusting the length of the antenna to bring the swr into a useable range, therefore you can get by without coax if you supply a proper ground system. The ground system is half of the antenna and is required for the antenna to radiate properly. But, endfed half wave antennas can be operated without a ground systme since they are voltage fed, not current fed, So there is no real reason to use a counterpoise on it, but it is smart to supply a good safety ground at least.