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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 04-08-2017, 5:41 AM
bms bms is offline
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Default Dipole fequency

Hi:

Can someone tell me what frequency is a 20-foot (total) dipole and what model LDG tuner would I use for a Yaesu 459D? Show me the math.
I have a Harbor Freight flagpole which is 20 feet tall and thought I would use a dipole inside the pole.
What gauge wire should I use?
Do I need radials or a ground?
How about a twenty-foot wire inside the pole and radials though I maybe have 10 foot around the pole?
Thanks.

crackerjohn
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Old 04-08-2017, 6:58 AM
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468 divided by the frequency in mhz. A 20 ft pole woulfd accomodate a 15 meters dipole, each leg about 9.61 feet from the center. The dipole would not require radials, though a vertical, you would need radials to work best. Cheapest wire is regular ac cord from the hard ware store. A vertical, you could put in wire for 20 meters, 16.7 feet, then at least two radials of 16 feet, 4 would be better. So for a dipole, 468/frequency in mhz. A quarter wave, half of the dipole, 234 divided by the frequency in mhz . A YT-100 tuner should get you by. If you don't need a auto tuner, drop by a hamfest and see if you can find a dentron jr tuner. Small, legal power tuner, has a meter. Manual tuners , once you set them for where yopu happen to hand outr, you can change bands and reset the tuners pretty quick.
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Old 04-08-2017, 9:23 AM
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A half wave dipole on 15m is over 22ft long and would not fit, it looks like the calculation was for 12m. You can also fit a 10m dipole inside.

I hope the Harbor Freight pole is fiberglass or plastic, you cant stick an antenna inside a metal pole and expect it to work.
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Originally Posted by teufler View Post
468 divided by the frequency in mhz. A 20 ft pole woulfd accomodate a 15 meters dipole, each leg about 9.61 feet from the center. The dipole would not require radials, though a vertical, you would need radials to work best. Cheapest wire is regular ac cord from the hard ware store. A vertical, you could put in wire for 20 meters, 16.7 feet, then at least two radials of 16 feet, 4 would be better. So for a dipole, 468/frequency in mhz. A quarter wave, half of the dipole, 234 divided by the frequency in mhz . A YT-100 tuner should get you by. If you don't need a auto tuner, drop by a hamfest and see if you can find a dentron jr tuner. Small, legal power tuner, has a meter. Manual tuners , once you set them for where yopu happen to hand outr, you can change bands and reset the tuners pretty quick.
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Old 04-08-2017, 2:07 PM
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Hi BMS
.
I think I have what you want to do as either a dipole mounted inside a flag pole ( I am assuming it is non-metallic) or to use the (non-metallic) pole to hold a vertical wire.
I am not familiar with the flag pole, but if it is metallic- forget both options.
.
However, that said, you can mount a dipole inside a vertical non-metallic pole, but you will have to run its feedline-- coax or what ever, off the center at a 90 degree angle, and run it for a considerable distance. It will not work other wise. If it’s a covert/hidden antenna you are trying to achieve, this will probably defeat it.

If you are versed in antenna designs you can construct a coaxial fed vertical by folding back the coax over the feedline, and placing it all within the vertical section of the mast-- but that topic is far beyond the discussion here.
.
Better still, is to run a heavy gauged wire up the inside of the mast, and use a tuner a the base. By using, as you mention'd, ground level radials, this 20 foot section can be resonated over a wide band of frequencies, at least 3.5, maybe 1.8, upwards to 30Mhz’s. Such a tuner can be manual, but a logistics pain- each time frequencies are change. Better, is an auto tuner, similar to those used in the maritime service…. Look at what Icom makes-- I use these for this very purpose- matching random vertical antennas....
But, and this is very important-- if you go this route-- mount the tuner **At the Base of the Vertical !**- tie it directly to your vertical radiator and the ground radials-- no coax etc., between it and the radiator !
.
.
Here is a simple formulae for calculating antenna lengths- let’s say- in your case: a 20 foot mast is approximately 6 metre’s. If you wanted a quarter wave vertical, this is ¼ of the desired wave length at ~24 metre’s -- and close to the 20 metre band.-- it could become a 20 metre band antenna. You will need to use more exacting calculations, however--
.
…ie: for a half wave dipole---
.
Length in feet = 491.8 x K/ freq Mhz.
.
………..K is determined by wavelength divide by radiator diameter… (in most cases 0.96 will do for HF dipoles)
.

.
I hope this points you in the right direction, BMS..
.
Good luck… further questions?.. Please ask away….
.
.
…………………..CF

Last edited by Coyote-Frostbyte; 04-08-2017 at 2:24 PM..
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Old 04-08-2017, 5:56 PM
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Push up aluminum pole per the Harbor Freight web site, the sections may or may not make good connection between them.
Feeding it at the bottom (assuming you can make an insulated mount) against a radial system and using most any antenna tuner rated for the power (I can't find a 459... maybe the 100W 450D?)
will work Although many bands are going to dissipate the power in the coax before it gets to the radiator. Better bet would be a weather proof auto tuner at the base of the flag pole.
There are lots and lots of instructions on the web for creating a flag pole antenna. It will be much more rewarding doing your own math and installation designs than getting advice from an open forum.

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Old 04-09-2017, 12:17 AM
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If you make a vertical with a tuner at the base, you're going to need lots of radials a quarter wavelength long at your lowest frequency - all bonded to the ground stake at the tuner
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