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Poll

Why is a dipole for 80 meters 120-some feet long while a mobile antenna for the same band is perhaps only 8 feet tall?

Dipoles are --by design -- the correct length
0 (0%)
verticals (and mobile antennas) are "compromise" antennas
0 (0%)
Dipoles don't require additional capacitance
0 (0%)
Dipoles don't require additional inductance
0 (0%)
All of the above
2 (100%)

Total Members Voted: 1

Voting closed: September 23, 2012, 03:32:40 PM

Author Topic: Dipole length vs. Mobile (auto/truck) height for same band  (Read 5905 times)

WA4STO

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Dipole length vs. Mobile (auto/truck) height for same band
« on: September 13, 2012, 03:32:40 PM »
I've been Elmering (mentoring) a fellow down in Florida (I'm in Nebraska) lately, and today I happened to ask him a series of questions to perhaps help him think about some of the gear he's been buying.

One of the gizmos he now has is a multi-band "fan" dipole.  The 80 meter portion is something more than 120 feet long.  So the question was:

"How come your dipole for 80 meters is 124 feet long while my vertical is probably only about 18 feet tall?  Oh, yes, my vert covers 80 meters just fine.  How can that be?"

Not sure if this graphic could be considered a "hint" or not.  In any event, it's a picture of my vertical antenna.






gil

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Dipole length vs. Mobile (auto/truck) height for same band
« Reply #1 on: September 13, 2012, 03:38:07 PM »
Is this a Butternut?

WA4STO

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #2 on: September 13, 2012, 06:22:21 PM »
Sure is, Gil.  It's an HF9V

The thing I like most about it is that the coils are filled with nuttin' but air.  Every other vertical I ever had utilized coils that eventually got wet inside or became utterly useless with time.

With this little (?) beastie, I sure can't complain about the quality.  Truly excellent!



gil

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #3 on: September 13, 2012, 06:41:54 PM »
What is your radials situation? I read that those verticals require an ungodly number of radials...

Gil.

WA4STO

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Ground radials for vertical antennas
« Reply #4 on: September 13, 2012, 08:08:01 PM »
You could NOT have asked a more pertinent question.

I think I was licensed for -- maybe -- 39 years before I really got the gist of why a radial system is critical.

I seem to recall the  manufacturer of the HF9V stipulating that 30 or 40 radials would be appropriate.  On the other hand, it's well known that commercial AM/FM transmitting antennas call for a "standard" of 100 radials.

Me, I put down about 22 as I recall.  I'm not happy with the way they are connected at the base of the antenna so I intend to fix that situation, as well as increasing the number of radials by a factor of two, prior to the Nebraska snows starting to fly.  Best laid plans and all that...
 
Two houses and a few years ago (still hadn't learned yet) I thought I was SO clever.  Had a vertical out in the back yard, mounted on a pole.  Base of the vert was about 6 feet up as I recall.  Put up about 3 radials that I thought might be resonant on 80.  Of course, THEY were six feet off the ground as well. 

I also had a multiband dipole, fed with open wire line (I feel another "poll" coming on...) that could hear all kinds of goodies on 80.  Not the vertical, though.  Huge difference between radials in an ABOVE ground system and radials IN the ground. 

Someone was likely saying something to me about being a "grasshopper".  Probably many someones.


gil

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #5 on: September 14, 2012, 12:35:51 AM »
I was thinking about getting a vertical, but decided not to because of the radials.. Kind of messy, and I rent.. I do have a Solarcon I-Max2000, which presumably is a "no radial" vertical, but we all know that there is no such thing.. I also like end-fed dipoles.. Same problem. The coax outer shield IS the radial! I can't put a dipole higher than maybe 30ft, and no longer than maybe 100ft. I might still try a doublet fed by ladder line. I am looking into magnetic loops, copper tubes, octagonal, 21ft circumference, 40m (55% efficient) to 20m (95%). Not sure about RF issues with that. I need to investigate more.. Of course the fact that I'll never use more than 15W might help not getting RF burns and since I can't really afford high voltage vacuum capacitors.. Still, I want most of my signal to radiate properly at the low power levels I want to use.. More about the mag loop in a future post...

Gil.

WA4STO

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #6 on: September 14, 2012, 09:58:45 AM »
That open wire feed line is SO useful.  I've likely had more antennas fed with various incarnations of that stuff than any other.

No need for me to preach to YOU, but for the benefit of the others here, it might be good to point out that open wire feed line isn't going to help that 100 foot (or less, maybe) dipole work on 80 meters.  Blood from a turnip and all that.

These days, I'm getting so lazy.  A few years ago, I was more than happy to twiddle the knobs on the antenna "tuner"; today I just wanna push the "tuner" button on the IC7000.  If I wasn't so lazy, I'd make sure that the vertical's bands were all tuned without the durned tuner!  Warm, fuzzies...

For the newcomers, here's a picture of open wire feedline.  There are several versions of it, or you can make it. 



If any of you would like to know why anybody would bother with this stuff, as opposed to using coax cable, just ask.  Might be a fun topic to banter around.


gil

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #7 on: September 14, 2012, 12:14:30 PM »
Well, I just ordered the capacitor for my magnetic loop, 9-110pf, 4KV. I'll probably get the tubing this week-end..

Gil.

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Re: Dipole length vs. Mobile (auto/truck) height for same band
« Reply #7 on: September 14, 2012, 12:14:30 PM »