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Author Topic: Dipole made from coax - possible?  (Read 1889 times)
WK5H
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Posts: 111




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« on: January 11, 2013, 01:32:19 PM »

I'm trying to make a semi-stealthy dipole for a rooftop, and after knowing that you can use 2 runs of coax together as semi-ladder line, it got me to thinking.

What characteristics would an antenna have, if made from coax, and the shield was separated in the middle, and tied to a run of "coax ladder line" back to a tuner?

The coax would be on top of the roof pitch, and would blend in, with the pitch being about 35ft off ground.

I don't have any antenna modeling software, and I though I'd run this by you guys first before I started waiting good coax.

Thanks in advance!
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WB6BYU
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Posts: 13482




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« Reply #1 on: January 11, 2013, 02:02:36 PM »

You can do that if you have plenty of coax to waste.

The coax would act like a fat wire:  impedances would be slightly lower than for a
thin wire, but overall performance would be basically the same.  It probably is
easier to hide a thinner wire than something fat like coax.

Whether there is any advantage in using coax to make the ladder line depends on
the circumstances and what short of feedline you need.  Using the two shields
as parallel conductors will give a relatively low loss line if they are separated
somewhat apart, but there isn't really any advantage to using coax in that case
instead of regular wire (which is usually cheaper).  If you are making "balanced,
shielded line" using the two coax center conductors with the shields grounded,
then the losses are the same as regular coax with an equivalent load impedance,
and losses may be high if used at a high SWR.

There is a dipole variant called the "Bazooka" that is commonly made with coax
where the center is opened (a better version cross-connects the braid and
center conductors in the middle) and the ends are shorted.  This has a little bit
wider SWR bandwidth and slightly higher losses.  There is a lot of ham lore and
myth about this antenna, but it doesn't work any better than a regular dipole.


So, yes, you can use coax to build a dipole and feedline, but unless you have a
pile of scrap coax that you are trying to find a use for, there won't be any
advantage to using coax in this application rather than plain wire.
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WK5H
Member

Posts: 111




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« Reply #2 on: January 12, 2013, 07:37:22 PM »

Makes sense.  Think I'll forgo wasting coax, and try to some other options.

Thanks for the response, and 73's!
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M6GOM
Member

Posts: 993




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« Reply #3 on: January 14, 2013, 01:44:53 PM »

Google Double bazooka dipole.
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G7DIE
Member

Posts: 65




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« Reply #4 on: January 14, 2013, 02:56:03 PM »

A very simple dipole can be made from coax, without using calculations just yet, if our dipole was 22' long (approximate length of a 15m band dipole), we could use a length of coax 11' long and a length of wire 11' long, we could attach the wire to the centre conductor of the coax and rely on common mode current for the return current of the dipole, that is purposly use the coax in an unbalanced mode with return current on the outside of the coax screen, the one proviso is that we choke the common mode current at the antenna base before we consuder feeding the antenna.

For example the antenna could be a the end of a 100' length of coax (if your shack was that far away) with just the last 11' acting as half of the dipole, if we used RG58 for a 15m dipole we could choke the antenna with ferrite beads or a torroid, 12 turns of RG58 on an FT240-61 should do the trick.

The whole antenna can be made from coax by removing the outer sheath and 11' of shield, but that just strikes me as wasteful Cheesy

Hopefully this makes sense, if it doesn't I'll hash a diagram together tomorrow, my work PC being hopeless for any kind of graphic work.
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ZENKI
Member

Posts: 980




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« Reply #5 on: January 14, 2013, 03:43:07 PM »

Dont waste your time using coax,  using coax achieves nothing that can be measured in the real world.

Use plain old copper wire. If you need a broadband antenna either use a fan dipole or cage dipole. There are broadband feed schemes around that allows  coverage of the whole 80 meter band.
I dont think its worth all the trouble. If it was me I would just antenna antenna tuner as a line SWR flattener for going up or down the band.

Modeling is not going to tell you much, coax dipoles dont have gain over wire dipoles! (unless you have curtain array of dipoles  on 300ft towers)

If the coax is old or gets a break in the jacket  it will soak up moisture and you will have a lousy antenna very fast.

What matters more is height above ground. You will gain much more by doubling the height of your antenna above ground than  playing around with crazy antenna schemes like coax
bazooka antennas. Since coax is so heavy you will be losing a lot of height   because of the droop. Just stick up a plain old wire dipole thats good enough. If you need more gain  at  the 35ft height get
a amplifier. 6 or 10db of gain from the amp is far more than you will ever get by playing around with bits of wire or coax.

Another thing that can be useful is using 2 dipoles  as a turnstile antenna. If you get creative you can switch
polarization and sense of the turnstile. Its amazing how somethings you can pick up 6 to 10db on receiver just selecting the right sense or polarization even from 2 low dipoles.

After the basic half wave dipole the next best antenna you can use is the double extended zepp antenna.  Feed the zepp with open wire line and you can use it on all bands.
If you just talking locally you dont want an antenna that reduces your beam-width. Although at 35ft high you wont have much of a directional pattern.

Another option that you might want to try is  vertical or an inverted L antenna. If you fed your low copper wire dipole with  open wire line you could short out the feedline and feed it against some radials and use the dipole
as a T vertical antenna. Dont believe people when they say vertical antennas are useless for talking down the road. You can open up  a band locally on 75 meters by just switching from a low dipole to a vertical. Not everything is high angle and not everything is low angle. You cant have too many guns in your armory!

You  have lots of options for other antenna designs that are proven. Unfortunately coax bazookas antennas are just voodoo antennas that offer no real advantage in the real world.







I'm trying to make a semi-stealthy dipole for a rooftop, and after knowing that you can use 2 runs of coax together as semi-ladder line, it got me to thinking.

What characteristics would an antenna have, if made from coax, and the shield was separated in the middle, and tied to a run of "coax ladder line" back to a tuner?

The coax would be on top of the roof pitch, and would blend in, with the pitch being about 35ft off ground.

I don't have any antenna modeling software, and I though I'd run this by you guys first before I started waiting good coax.

Thanks in advance!
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