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Author Topic: mobile antenna power rating  (Read 4420 times)
KJ4YZI
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Posts: 38




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« on: March 22, 2012, 12:32:29 PM »

JUST A QUESTION...............if a mobile antenna says handles 100 watts, could i rewind the small winding coil in the center of the whip to increase the power rating, of course using a larger gauge wire in the coil?
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KB9BPF
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« Reply #1 on: March 22, 2012, 01:10:10 PM »

In short, yes.

Of course there are many factors that influence the inductance requirement (find and read a copy of Don Johnson's Mobilleer's Handbook to learn more) but as long as your whip length has some adjustment to it you should be OK. Just make sure your new coil has the same inductance as the old one.

If you don't have an LCR meter, then you can get pretty close by carefully measuring the original number of turns, coil length, and coil diameter then calculate it with the formula for solenoidal wound coils that you can find easily in a Google search.

Then run the numbers on the new coil you intend to wind and make adjustments until they match pretty closely. I'd shoot for one or two percent error, but it can be sloppier if your have plenty of whip length adjustment. I advise you to err a bit on the side of higher inductance because it's easier to cut a little off the whip to compensate than to add it!

If you can replace the whip with a longer one then you might be able to use the original form and less inductance. The less inductance you use the higher the efficiency, everything else being equal. There are equations in the ON4UN low-band DX handbook that you can use to get an idea how much inductance you'll need if you vary from the original whip length.

Remember that with bigger wire the form will not fit as many turns and you'll need a bigger form to get the same inductance. Be careful not to let the form be too long and thin. Much more than 3:1 (length/diameter) and your efficiency will start falling off fairly rapidly.

When I was first licensed I rewound an old base-loaded CB antenna for 2M. Worked great.

When I first got into in HF mobile I went through the obligatory 'Hustler' stage. Bought up used 75/80M and 40M resonators at hamfests for cheap, stripped the thin wire off and rewound them with #14 enameled, still closewound. Was able to make the 75/80M resonators into decent 40M resonators and the 40M resonators into decent 20M resonators.

Good luck and have fun! I enjoyed my experiments with rewound Hustler coils immensely and learned a lot. Eventually I started homebrewing my coil forms when I had the good fortune of aquiring a lathe.

73
Brad
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W8JX
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« Reply #2 on: March 22, 2012, 04:22:02 PM »

JUST A QUESTION...............if a mobile antenna says handles 100 watts, could i rewind the small winding coil in the center of the whip to increase the power rating, of course using a larger gauge wire in the coil?

As mentioned you can rewind but you need to change diameter of coil and wire size to increase power handling/dissipation rating of it. Not just a simple rewind.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
N6AJR
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« Reply #3 on: March 23, 2012, 10:44:08 AM »

But normally the limiting factor in mobile work is not the transmitting, but the recieving.  If you can't hear them , no sense having a zillion watts, whrn a mobile antenna is usually only good for hearing  a minimal distance. and if the band is in, 5 watts is enough.
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W0FM
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Posts: 2057




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« Reply #4 on: March 23, 2012, 11:03:11 AM »

An antenna "rating" usually suggests a publicized claim by the manufacturer.  In my experience, the claims are typically based on one of the following:

1.  Technical calculations.
2.  Field testing
3.  Technical Calculations AND Field Testing.
4.  SWAG  (Silly Wild-A**ed Guess)
5.  Flat out marketing lies.

It's up to us to determine which of the above the manufacturer used.  Generally, I would tend to believe something to the lower side of any power claims in literature and advertising because anything other than numbers 4 and 5 require some financial outlay by the manufacturer.

73,

Terry, WØFM
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W8JX
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« Reply #5 on: March 23, 2012, 12:32:52 PM »

But normally the limiting factor in mobile work is not the transmitting, but the recieving.  If you can't hear them , no sense having a zillion watts, whrn a mobile antenna is usually only good for hearing  a minimal distance. and if the band is in, 5 watts is enough.

Actually I do not find that the problem but rather them hearing you. I will generally not even call a station when mobile that is not at least 57 or better. I have learned that at that level they can usually hear me as well.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
K0BG
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« Reply #6 on: March 24, 2012, 04:11:47 AM »

If you really want to have some fun, beg, borrow, or steal an IR imaging camera. Things work best if it is a cool night, and you've allowed the antenna to come to ambient temperature. You dump the advertised power into the antenna for about 30 seconds to a minute. Sometimes, it doesn't even take that long. You can sure tell where the losses are, and they're usually where you least expect! Which ones heat up the fastest? Take a WAG.
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K5LXP
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« Reply #7 on: March 24, 2012, 06:30:01 AM »

If you really want to have some fun, beg, borrow, or steal an IR imaging camera.
...
You can sure tell where the losses are, and they're usually where you least expect! Which ones heat up the fastest? Take a WAG.

In the case of the Tram dual band antenna, you don't need the IR camera.  Just put 50 watts into it for a couple minutes and the whip will turn into a coil "melt indicator" as it leans over.   I'll add one more power claim to W0FM's list, "watts per mph".  The faster you're going, the more wind you get to keep the coil cool.


Mark K5LXP
Albuquerque, NM
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WX7G
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Posts: 6321




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« Reply #8 on: March 24, 2012, 07:49:05 PM »

I ran a 40 meter Hamstick with a 400 watt amp in OOK CW mode. The average power was 200 watts and during long ragchew QSOs the antenna made a good loss indicator. It would smoke while driving 70 mph.
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K0BG
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« Reply #9 on: March 26, 2012, 10:26:27 AM »

I recently had a young man tell me he was running an ALS-500 into a hamstick without any problems. It was mounted on a K400. When he remounted it on a ballmount, it went up in smoke. Just goes to show you how important it is to keep ground losses low.
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KJ4YZI
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Posts: 38




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« Reply #10 on: March 27, 2012, 11:03:08 AM »

wow, a lot of info. i can say that when i transmit for a long distance driving, and park and feel my antenna, it is not hot at all, that is 160 watts into a 7/8 wave antenna and the antenna is stamped boldly, 200 watts handling" but the website now claims 150 watts, "the sum of both bands being operated simutaniously" so i guess this antenna is rated for 100 on vhf and 50 on UHF... oh well
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W0FM
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Posts: 2057




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« Reply #11 on: March 27, 2012, 12:42:56 PM »

Since the subject has shifted to heat here, I'll relate one of my favorites.  In the late 70's while working for Motorola, 800 MHz FM mobiles had just been introduced.  Some Motorola engineers were in town and we all went to a "business men's special" Cardinals game for some R&R.  We were tailgating in the parking lot before the game.  My company car had a brand new 800 MHz quarter wave whip installed in the center of the roof and a 35 Watt 800 MHz mobile in the trunk.  One of the engineers grabbed a brat out of the cooler and "skewered" it longwise down on the antenna.  He keyed up my mobile and we all watched in amazement as the "vertical" brat began to sizzle, bubble and smoke on the car roof.  We instantly understood why these engineers had highly discouraged us from using the "new" through-the-glass cellular antennas with these 35 Watt mobile radios. 

Can you spell "microwave"?

73

Terry, WØFM
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WB6BYU
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Posts: 13572




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« Reply #12 on: March 27, 2012, 04:07:32 PM »

Quote from: KI4MQJ
... the website now claims 150 watts, "the sum of both bands being operated simutaniously" so i guess this antenna is rated for 100 on vhf and 50 on UHF...


Only if you are transmitting on both bands simultaneously.  Otherwise it should handle
150 watts on EITHER band, just not both at once.

I can't think of any situation where I'd want to be transmitting on both bands at the same time,
especially at full output power.  Even something like a cross-band repeater is only transmitting
on one link at a time.

So as long as you have 0.1dB of feedline loss, you are within the 150 watt power rating
of the antenna if you only transmit on one band at a time.
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NEVBEN
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Posts: 43




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« Reply #13 on: March 29, 2012, 10:23:14 AM »

Since the subject has shifted to heat here, I'll relate one of my favorites.  In the late 70's while working for Motorola, 800 MHz FM mobiles had just been introduced.  Some Motorola engineers were in town and we all went to a "business men's special" Cardinals game for some R&R.  We were tailgating in the parking lot before the game.  My company car had a brand new 800 MHz quarter wave whip installed in the center of the roof and a 35 Watt 800 MHz mobile in the trunk.  One of the engineers grabbed a brat out of the cooler and "skewered" it longwise down on the antenna.  He keyed up my mobile and we all watched in amazement as the "vertical" brat began to sizzle, bubble and smoke on the car roof.  We instantly understood why these engineers had highly discouraged us from using the "new" through-the-glass cellular antennas with these 35 Watt mobile radios. 

Can you spell "microwave"?

73

Terry, WØFM


I bet that tailgate was a real doozie.  Did you talk about your coax?

You can take a bunch of nerds to a ballgame, but they're still a bunch of nerds!

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W0FM
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Posts: 2057




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« Reply #14 on: March 30, 2012, 01:18:26 PM »

Only two were engineers.  The rest were "quasi-technical" sales types.  But we did comment on the new low-loss Teflon coax on the mobile antennas.  Mostly because we had never seen white coax prior to that.   Grin

Terry
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