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Author Topic: Mobile antenna; efficiency and physical length  (Read 2627 times)
AC4RD
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« on: April 14, 2011, 09:15:19 AM »

I've looked on K0BG's excellent site under "Antenna efficiency," and this statement is in the direction I'm headed in:  "It should be noted that the increase in Rr (when electrical length is increased) is due to the current node (point of maximum antenna current) being modified. If you need more information on this, do a Google search for degree amperes. Or better yet, read this page."    But the link doesn't appear to be working, and rather than wade through dozens of web pages, I'll take the easy route and ask the smart people.  :-)

I'm playing on HF from the car, mostly 20 and 17 meters, with a severe height limitation on my antenna(s).  I have to park in a garage at work, and the antenna has to fit inside the garage, I'm not going to get out and fold antennas over/etc.  There's no place to do that near the garage that doesn't block traffic.   How low is the garage roof?  Inside the garage, I can touch the lower beams with my hand, standing flat-footed on the ground. 

So I need, obviously, a SHORT antenna for HF.   Right now I'm using Hustler resonators on the 36" mast from DX Engineering, because the standard 54" Hustler mast won't fit in the garage.   This is on a bumper-height mount.   Lately, I've found myself wondering if I should take off the 36" mast and replace it with the 54" mast on the weekends, when I don't have the restriction of the garage to deal with. 

So my question is:  Is the extra 18" of mast length enough to make any significant difference in antenna efficiency?  I'm guessing it isn't--that the antenna is already so short that another 18" of mast won't be noticeable.  But I'd be interested in knowing what the numbers are, if anybody can answer that.   Thanks!  --Ken AC4RD
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W3LK
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« Reply #1 on: April 14, 2011, 11:54:40 AM »

I have to park in a garage at work, and the antenna has to fit inside the garage, I'm not going to get out and fold antennas over/etc.  There's no place to do that near the garage that doesn't block traffic. 

I have to fold over the 6' whip on my High Sierra 1800 when I enter garages. It takes all of 20 seconds to get out, fold over the whip and get back in the vehicle. Never had a problem holding up traffic at the entrance while doing it.

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K0BG
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« Reply #2 on: April 14, 2011, 04:39:28 PM »

I'm sorry about the link. I regularly check them with a validation service, but once in a while, one sneaks by.

The radiation resistance of a vertical antenna follows a square law function. That is to say, double the length, and the radiation resistance increases by four. And, the amount of inductive reactance required to cancel out the capacitive reactance short antennas (less than 1/4 wave length long) exhibit, also decreases. This is why I state that length matters.

Pundits of short, stubby antennas will always argue that the difference is small (<3 dB). In reality, it is more like 6 dB, and perhaps a good deal more depending on where and how the antenna is mounted. This said, what really counts is how much signal to noise ratio you can generate in the front end of the transceiver on the other end. Sometimes, 1 dB increase in radiated power can make a huge difference. You can accomplish this by radiating more power, adding an amp, or decreasing the ground losses (better mounting). You have to decide, just how far (read that as expense) you're willing to lay out to get where you want to be.

Speaking only for myself, I spare no expense!
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N5UD
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« Reply #3 on: April 14, 2011, 08:48:13 PM »

I have played with a 3 foot whip above my screwdriver coil for 10 meters. Versus a 7 foot whip and cap hat above the coil. My total height above ground is 13 feet with this arrangement. 10 miles from me is a 10 meter beacon. There is a 4 S unit increase using the longer configuration on 10 meters receiving that beacon. The background noise comes up too. This is on my receiver, and is repeatable. May not be the same on another receiver.

I would have to assume the same increase would be shown in my transmitted signal too.

Dave N5UD
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K5DUT
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« Reply #4 on: May 31, 2011, 06:29:15 PM »

Yes the length is important. It carries high current. The longer the part of an antenna  that carries high current, the more efficient the antenna is.  A bumper is the least efficient mounting place on a mobile due to ground loss.  At least one company makes a motorized layover assembly for antenna's that you can operate from in the vehicle.  That device with a roof mounted antenna is about the best you can do.  A few years ago, there was an antenna modeling  program available  from the ARRL  called "Mobile".  Its good for H.F. mobile antenna's and allows the user to program in and change just about everything that's important to performance.  Its a good tool to use when designing H.F. mobile antenna's.

Don K5DUT
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N8BHL
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« Reply #5 on: June 05, 2011, 07:25:32 AM »

Ken, I had the same problem with my mobile antennas- I have several antennas mounted on the hood on either side of the windshield. A couple are mag-mount for scanners, and there's a dual-band with a clip-on mount (Diamond- no trouble at all). I had a collection of hustler coils and a foldover mast, so that was the default choice for my Dodge Ram pickup. But in order to work well on HF, they're all too high for garages and such. Honestly, I just wasn't interested in getting out of the truck to raise the foldover mast, then reversing when I arrived at my workplace garage. So despite the warnings of Alan, K0BG (guru-level and nice guy) I went with the short Hustler mast, which placed the coils right at the level of my pickup bed. I am glad I spent the bucks for a bumper mount from Breedlove Machine Shop. 80 Meters brushes the low garage spots a bit, all the other coils are much lower than the garages and fit just fine. I normally leave 20 on. I can report that while I'm not optimum, (and I don't know what all I am NOT hearing) I can be heard very well during openings on 20, with contacts all over Europe, Croatia and elsewhere. So in overcoming the "tall mast" problems, I've been very pleased with the results.   
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AC4RD
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« Reply #6 on: June 05, 2011, 08:37:26 AM »

..they're all too high for garages and such. Honestly, I just wasn't interested in getting out of the truck to raise the foldover mast, then reversing when I arrived at my workplace garage. So despite the warnings of Alan, K0BG (guru-level and nice guy) I went with the short Hustler mast, which placed the coils right at the level of my pickup bed ... So in overcoming the "tall mast" problems, I've been very pleased with the results.   

N8BHL, it's the same for me--I'll raise the antenna leaving the garage on Friday, and put it back lower when I get to work Monday, but even that is a bit of a nuisance.   I'll tell you what I'm using right now, and it seems to work pretty well for me, though you're going to laugh.   I've got a taller mast, a couple of springs, an adjustable angle mount, and the longer whip (which seems to make a big difference in performance; around 38" of on the 15m resonator makes it play nicely on 20 meters).  This antenna is about 3" too tall to fit in my parking garage, when it's straight.  But during the week, I leave the resonator bent over around 45-60 degrees from vertical--sort of a lazy L shape.   It looks pretty stupid, but it SEEMS to work better than the short versions that fit into the garage.  (I haven't done any direct comparisons with the bent version, though the straight version is clearly more efficient than its shorter version.)   Anyway, it's not a good long-term solution:  It looks stupid, and sooner or later the angle adapter will be too loose and I'll have the resonator/whip flopping around loose on my drive ... but it's working, and I have fun on my drive to work, so I'm reasonably pleased with it for now!  :-)
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