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Author Topic: Mobile Antenna Thought  (Read 400 times)
N4ZYV
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Posts: 90




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« on: July 28, 2004, 11:24:59 PM »

I'm thinking of installing an HF radio in the little pickup and have a thought about the antenna and I'd like to run it by you guys.

I really have no experience with HF mobiling except for 10 meters. For that, the old stainless whip on a ball mount always worked well. Now though, things will get a bit more involved.

From what I have read here and elsewhere, the way I plan to go is with an auto-tuner and a whip. Simple, works well enough. Problem (as I get it from the forums) is that a 9' whip is barely adequate for a whip but anything taller really becomes troublesome.

So, here is my thought on the short whip problem. How about installing one whip on a ball mount (or something else that may be suggested and work better for this) and feed the antenna there, then mount another whip on the other side of the truck and weld a rod between the tops of the two whips. This would give you a much longer whip and I would expect it would radiate more signal and be easier for the tuner to match on the lower frequencies. The "other" whip would not have any coax attached to it. The mount on that side would be there just to support it. So, one whip on the passenger side, a (stainless steel) rod welded on the tip leading to the driver's side of the truck and welded onto the tip of another whip over there. Combined length would be about 23'. 9' up one side, about 5' across the top, and another 9' down the other side.

Seems the added length would make matching the lower frequencies easier for the tuner and the added metal in the air should radiate (and capture) more signal.

Any opinions on this?
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KB9YNB
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Posts: 115


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« Reply #1 on: July 29, 2004, 12:10:31 PM »

I swear this EXACT same topic has been discussed before on this very site.... searching ....

http://www.eham.net/forums/MobileHam/3404

I just searched for "weld" and let my super-powers do the rest.

It could be done, but I don't think you'll like the result.  I'll also beat Mr. Applegate to the punch.

visit www.k0bg.com for good mobile installation tips.

KB9YNB
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K7VO
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Posts: 1010




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« Reply #2 on: August 01, 2004, 04:21:44 PM »

Look into some of the multiband (HF, 6m, 2m, 70cm) antennas by Comet, Maldol, or Diamond.  I know a number of people who have had great results with them with just a Diamond K-400 trunk lip mount or equivalent.  Of course, drilling a hole and getting the best possible ground is better, but I won't do it.

I will be going with a Maldol HS-6S myself once my Patcomm PC-9000 arrives.  That will be mainly used as a base rig, but I intend to use it mobile with the Maldol antenna on long trips.  I'll post and let people know how it works out.

73,
Caity
K7VO
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KE4SKY
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« Reply #3 on: August 03, 2004, 11:02:51 AM »

For my first HF mobile operation I tried a Hustler mast on a ball-mount, with triband adapter and three resonators for 20, 40 and 75 meters.  While this enabled easy band switching on the road, the performance of the Hustler coils left alot to be desired.  Hamsticks mounted high on the vehicle with a good ground were at least a full S-unit better than the Hustlers on both Tx and Rx.  But if you want a multi-band antenna with good performance, one of the screwdriver designs is probably the way to go.  

However, I needed to have something inexpensive, tree bashing rugged for back roads, and easily removed, theft resistant, for the urban work QTH and indoor garages, so I migrated to using hamsticks on quick disconnects, on a Diamond K400-3/8 hatchback mount.  I leave the 40 meter stick on the Cherokee most of the time, but carry 10, 20, 60 and 75 sticks with me to enable changing bands with propogation or to make a net sked.  This works for me.

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WA4UF
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« Reply #4 on: August 03, 2004, 02:02:10 PM »

I've become quite the bugcatcher fan, despite the inconvenience of having to stop, get out and move the jumpers to change bands.  Take a look at the "$20 Martian Death ray" homebrew bugcatcher in the April 2000 QST.  If you've got some stuff in the junkbox (and what ham doesn't?) and a nearby hardware store, you've got 20m-6m covered for around $20 to $30.  It's proven plenty rugged on my car, and heck, it's cheap enough and easy enough to build that if you *do( lose one to low-hanging branches it's not that big a deal.  One tip: RadioShack doesn't sell the 39" whip any more; I picked one up from an antenna parts vendor at a hamfest for $5.
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AA4PB
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Posts: 12769




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« Reply #5 on: August 03, 2004, 03:09:43 PM »

Short whips on the lower bands result in a very high impedance feed point which in turn results in some quite high voltages at the feed point. The key to success with a short whip is to use a large insulator (not the typical ball mount) to handle the high voltages and minimize capacitance between the vehicle and the whip mount. Mount the whip in a location that is clear of vehicle metal in order to minimize capacitance. The bumper for example, is generally a poor location because it leaves much of the whip running close to the body. The tuner should be mounted very close to the whip and connected with a single wire feed as short as possible. Never use coax between the whip and the tuner as it will have a lot of capacitance and bypass the signals around the antenna.

One option for better efficiency on 75M might be one of the long MFJ telescoping whips that could be used in lieu of the 102-inch whip while you are parked.
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N4ZYV
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Posts: 90




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« Reply #6 on: August 04, 2004, 11:05:08 PM »

Kb9YNB, ya that was me. I thought I had asked this before here but couldn't find it. So, I asked again. Doesn't matter anymore right now, the engine in the truck blew. Sad No money for a multi-band HF rig now.

It boils down to me reading about the low efficiency of the short whips that have to be used on a vehicle. Both screwdriver and whip-and-tuner antennas are mostly limited to a 102" whip. Power lines, trees and low flying aircraft are all concerns when contemplating a 20' whip on your trunk mount. Seems to me a longer whip would help out on the lower bands. It also seems to me the whip doesn't need to be straight vertical. Perhaps for optimum radiating pattern it does, but not simply for increased signal out and less wasted as heat inside the tuner or screwdriver coil.

In the other thread, I was told to build it and see how it works. No problem with that but how can I tell if the tuner is having an easier time of matching the antenna on the lower bands? How can I determine if there is any real usable increase in effeciency? Would simply taking field strength measurements show that? Is there an antenna modeling program that could crunch the numbers on such an odd antenna?

I've been told using a short whip on the lower bands creates high voltage at the feed point and was advised to use something with better insulating properties than a simple ball mount. Well, wouldn't the better-than-100% increase in whip length help out in that regard? Alleviating some of the problems with a short whip on the low bands is the reason for doing this in the first place.

I don't want to use a screwdriver antenna because one of the complaints I read about them is the low efficiency caused by the short whip on the low bands. This much longer whip should help in that regard.

As you guys can see, I just don't know much about antennas. I just keep reading about how poor the 102" whip is on the low bands regardless of whether its on a tuner or in a screwdriver. I know that on a vehicle, there really isn't much that can be done due to the extreme space limitations, but this seems to be easily done and is, at least, something.

Hey, how about running a coil of wire around the roofline of the fiberglass topper? Turn my truck into a big, radiating capacitor? Wink Ok, I'm getting silly.........
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K4KWH
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Posts: 51




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« Reply #7 on: August 14, 2004, 10:24:47 PM »

Generally, the less coil you use, the less loss there is. I wouldn't rule out the screwdriver, tho.  They work pretty well within the confines of known mobile antenna theory. I will tell you this, tho.  A screwdriver with, say-----a 66" whip will blow away a tuner/whip combo no matter how long a whip you use! All a "tuner" does is present a 50 ohm match and absorb heat! Heck, you can load a 50 ophm resistor, but you won't have much of a signal!Particularly, on the low bands.  While you will be down in the soup with the whip, the guy with the screwdriver will likely be quite readable.  Of course, you can run a 102" whip on a SD. And, yes, it gets dicey, if not illegal, when you start bustin' stoplight bulbs!  I like to use my SD with a long wire sometimes. Just sling a wire and an insulator over a tree limb and go.
Makes a great Field Day setup!  

Now don't circle your wagons on me. ;-) To each his own, and everybody has their own take on the "best" antenna.  Many screwdrivers perform excellently, and
some folks like "tuners" for their own reasons.

73
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