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Author Topic: Helical Vertical  (Read 2654 times)
2E1CLS
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Posts: 23




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« on: December 11, 2011, 06:00:20 AM »

Hi, I have built a Helically wound vertical using a 10m telescopic fibreglass pole and a 9.1 unun at the bottom with several lengths of counterpoise dug under my lawn. Now my question(s) is...

Is a 9.1 unun be the best choice or should I be using something else? Like a 4.1 unun or some say a balun but this being a unbalanced antenna surely needs a unun. Btw I know a tuner at the feedpoint would be best but I dont want to pay that much as this is a cheap build antenna and I intend to keep it that way :-)
Also I made the antenna Helically wound to fit a longer length of wire on the 10m pole for better results with 80m to. Now I'm not to fussed about loosing that gain on 80m if it means loosing some of the winds on the pole to better suit 40-20m? Or will this not affect these band at all?

Sorry if the questions sound a it obvious but this is a bit of a learning curve for me!

73s Carl M6CLS.
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2E1CLS
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Posts: 23




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« Reply #1 on: December 11, 2011, 06:04:19 AM »

Btw the antenna is ground mounted!

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W8JI
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« Reply #2 on: December 11, 2011, 06:24:53 AM »

The length of wire wound in an area does not mean you have a longer antenna. It just means you have changed the loss and current distribution in the area of antenna you already have.

Because the  distributed capacitance and inductance, and the distributed loss resistances, all change by how the antenna is constructed, no one can predict anything. The results will vary wildly with construction details, even including insulation thickness.

It is a very random result, not at all like a straight conductor running through space.

73 Tom

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G3TXQ
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« Reply #3 on: December 11, 2011, 06:31:54 AM »

Carl,

It's impossible to give you an accurate answer about your choice of UnUn without knowing the feedpoint impedance of your vertical on the various bands. Helicals are difficult to model, so measuring the impedance with an analyser is probably the best way to find out.

My guess - nothing more - is that the 9:1 UnUn will produce less overall loss on some bands, and the 4:1 less loss on others; but overall there will be no clear winner. I did some experiments on a similar arrangement here:
http://www.karinya.net/g3txq/unun/

73,
Steve G3TXQ



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W5DXP
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« Reply #4 on: December 11, 2011, 06:54:24 AM »

Hi, I have built a Helically wound vertical using a 10m telescopic fibreglass pole ...

At 1/4WL resonance, the feedpoint impedance will probably be around 20 ohms. A 9:1 unun will change the SWR from 2.5:1 to 22.5:1. Why would one want to do that?

Above their self-resonant frequency, helical antennas are notoriously lossy and do not make good multi-band antennas.

If you plug your helical into the following inductance calculator, it will yield some interesting information, like the self-resonant frequency and the losses at the different frequencies:

http://hamwaves.com/antennas/inductance.html

This metric/English conversion page will make things easier:

http://www.worldwidemetric.com/measurements.html
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
2E1CLS
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Posts: 23




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« Reply #5 on: December 11, 2011, 06:59:23 AM »

Ok thanks guys,

I have no access to a Analyser sadly.

Would you say in this case I may be better with the wire coming straight down the pole?

Thanks Carl.
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WB6BYU
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Posts: 13288




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« Reply #6 on: December 11, 2011, 09:00:01 AM »

Quote from: M6CLS

Would you say in this case I may be better with the wire coming straight down the pole?




That depends what you are trying to accomplish.

The 10m straight wire should work well on 40m without any matching.  On 20m it will be a
high impedance:  it can work well, but the SWR will be high on the coax which will increase
losses significantly.  SWR will also be high on 80m.

The common 43' vertical and similar designs intentionally choose a non-resonant length to
avoid the high impedance of a half wave radiator.  For example, 9m might be a better length
for 14 MHz (and 28 MHz) than a 10m wire without losing much performance on 7 MHz, though
both will be rather short for 3.5 MHz.

Another solution would be to run two wires up the mast - a straight one for 40m, and a second
with a trap or loading coil in the center designed for resonance on both 20m and 80m.  That would
give you a low SWR on all three bands.


As others have mentioned, helical antenna design is not just  a matter of winding some wire on
a former, and the resonant frequency isn't determined by a convenient formula based on the
total wire length as some might like.  The Hamwaves inductance calculator (linked to earlier) is
probably the best way to check a design, but it assumes a constant winding pitch.  Helicals with
significant shortening are often inefficient other than on their design frequency, though I've seen
designs that use a tapered winding (close wound for the top 1/3 of the antenna, with spacing
increasing towards the base) which was supposed to give better multi-band performance.  Even
that was based on a specific wire gauge and former diameter, and can't necessarily be generalized
without further experimentation.
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WX7G
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Posts: 6079




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« Reply #7 on: December 11, 2011, 12:15:45 PM »

I would feed the antenna direct with coax to a tuner. No balun or unun is needed.
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K0ZN
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Posts: 1548




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« Reply #8 on: December 14, 2011, 06:29:15 PM »


 It sounds to me like you are trying to feed a SINGLE wire, helical or otherwise, on multiple bands. Keep in mind there will be only a very few specific
frequencies where that will present a "reasonable" load impedance to the coax. To wit: when the antenna is resonant as a 1/4 wave monopole radiator and
at ODD harmonics. On all other bands/frequencies the impedance will vary radically and essentially be a terrible match to 50 ohm coax.

Bottomline:  Helical verticals can be effective, but they are essentially SINGLE band antennas.

If you want to feed that antenna on all bands, basically, it would perform better as a single wire, however, it will not present a good radiation pattern
once the length of the wire goes over 5/8ths of a wave length.  Bluntly, you have several compromises and you need to choose what bands
you want it to work best on.

Respectfully, pick up a copy of the RSGB Antenna Book or the ARRL Antenna Book and put in a couple of hours of study on antenna basics, especially
those regarding vertical antennas. This whole subject is much too complex to be adequately addressed in a few paragraphs on a website. The graphics
presented in an organized manner are the key to understanding. The time you put in picking up knowledge will pay off many times over in better signals and
enjoyment of the hobby.

73,

K0ZN
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GOUDURIX
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Posts: 206




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« Reply #9 on: December 19, 2011, 09:07:59 AM »

I use a 9,2m vertical antenna (9,2m wire along a 10m long glassfibre fishing pole - I insist glass fiber not carbon like most fishing poles today), with 4 elevated radials 1/4 lambda for 40m-20m-17m-15m (in total 16 radials but combined in 2-strand wires). A CG-3000 atu (rather cheap but oK still $300, a 1:1 choke at the feed point, the base is at 5m high.
I use and old stretch of RG-58 coax, on the the outer mantle, as a slightly thicker radiator.
9,2m seems to be a good length not giving the atu a hard time to tune, even on 20m.

This tunes and works well for DX with low take-off angles 80m-15m (above 21 Mhz the wire is > 5/8 WL hence shooting vertical lobes to the sky).

The only expensive item is the ATU.

I had an air cored 1:1 balun (10 turns of the RG-8 coax wound on a 4-1/2 inch pvc tube) but this is surely not choking well on 80m and I suspect some nasty resonance effect on 20m because I lose a lot of RX signal on 20m.

Jan
ON3ZTT

If you have open space in your garden, you can easily install it on a 3m pole, guyed.



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