Dipole theory adoption.

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nikharr
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Dipole theory adoption.

Post by nikharr »

I have a problem with installing an antenna on a plastic bodided car.

My first attempt, initially gave good swr results, but the reading over time fluctuated.
This is what I tried first, I had a Sirio ML145 mounted on the roofbar, the ground side I ran some single core 13a wire to the floor pan.

I then installed an 1" coil about 10" long of the same wire in line to the floor pan. (the coil was hidden in the door pillar)
This gave a good swr but didn't last long.
So I tried a second coil to the oposit side of the car and also connected it to the floor pan.
Same result again, initially good but didn't last long.

My line of thought now is perhaps to try and treat it like a dipole.
The antenna is suposedly a bottom coil loaded 5/8 wave antenna.
If I work out the dipole length for a given frequancy and base it on 5/8 wave, I would theoretically have the ground wire length?

Would this work, would it need to be a length laid through the car or could I wrap it around the roof bar. I have some spare insulated dipole wire I could try.

Any thought would be appriciated.
I am thick skinned so dont worry about comments.
Its just an thought and would like to hear from more experianced people than myself.

Thanks Nik.
jazzsinger
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Re: Dipole theory adoption.

Post by jazzsinger »

Your ml145 isn't 5/8 wav,thats just manufacturers misleading people to sell antennas,its a coil loaded 1/4 wave.if you want to improve the ground plane use wres around 9ft long,make sure they are waterproofed at the connecting points,as corrosion is probably whats causing your fluctuating readings,either that or your measuring when close to metalic objects.

even if it was a 5/8 most of the best full size 5/8 waves use 9 ft (1/4 wave) radials.
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Adriano9966
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Re: Dipole theory adoption.

Post by Adriano9966 »

:D erm this is just a thought that might solve the problem
many years ago I had a friend called wurzel on the cb who drove a reliant which also had a plastic body he got around the problem by placing a piece of sheet metal under the roof head lining and then earthing this to the chassis of the vehicle using copper wire it was a very neat job indeed and did not look obtrusive with his dv27 ;) The down side was having to drill a hole in the body of the vehicle

I also grew up in the Sratford upon avon district and this arrangement I note was also used by a number of friends on thier fiberglass boats to great effect if memory serves there were also a number of cb antennas produced for this purpose that dont require any ground plane eg there is an antenna with artificial groundplane produced for trucks with a fibreglass cab

:D hope this helps
nikharr
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Re: Dipole theory adoption.

Post by nikharr »

Thanks for the advice,

below is a quick sketch of an idea would this work?
The antenna mounted on roof bar, then wire run across the roof bar either side.
Then into the car and large loops front and back under the roof lining.

Thanks again Nik.
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Re: Dipole theory adoption.

Post by JamesA »

M6JCA -> 2E0JCA

Yaesu FT-897, FT-857D (In car), FT-817D & Wouxun KG-UVD1P
Full wave 40M wire loop for HF, Watson V2000 for 2, 70 & 6
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Re: Dipole theory adoption.

Post by jazzsinger »

nikharr wrote:Thanks for the advice,

below is a quick sketch of an idea would this work?
The antenna mounted on roof bar, then wire run across the roof bar either side.
Then into the car and large loops front and back under the roof lining.

Thanks again Nik.
rather than have loops under the roof lining,i would make a 1 ft gap in between the loop at back and front so it is only 2ft either side,

that would give you 4 x 2.5 ft+4.5ft+2ft = 4 x 9ft,which would equate to 4 resonant 1/4 wave radials.
just an idea for you.its been proven that bending radials doesn't affect efficiency too much.
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jazzsinger
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Re: Dipole theory adoption.

Post by jazzsinger »

JamesA wrote:This any use?

http://tinyurl.com/yext8sb
they don't work,they coudn't possibly improve swr as they are either connected to the radiating element or connected to nothing at all,not to mention they are far too short to be efficient.definately snake oil.

the only way you could add a radial kit like that is on a so239 type fitting which has both a ground and radiator connection with it connected to the ground one,but again lack of size of the radials would prove highly inefficient at 27 mhz,much better at vhf/uhf.
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nikharr
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Re: Dipole theory adoption.

Post by nikharr »

Cheers George,

But I am not sure what you mean, would you be able to post, pm or email me a sketch.
Bear in mind that the 2ft wire runs from the antenna are on the roof bar before they enter the car via the rear door openings.
Drilling a hole in the roof is not an option.

Thanks Nik.
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Re: Dipole theory adoption.

Post by jazzsinger »

nikharr wrote:Cheers George,

But I am not sure what you mean, would you be able to post, pm or email me a sketch.
Bear in mind that the 2ft wire runs from the antenna are on the roof bar before they enter the car via the rear door openings.
Drilling a hole in the roof is not an option.

Thanks Nik.
hi Nik,

I'm not suggesting you cut a hole,merely that you insert a gap in the wire loops so that each of the four radials would be 1/4 wave resonant.it would be fine to route them the way you planned.hope the high quality artistic impression,(pmsl) helps you out,as you can see art isn't one of my strong points but i'm sure you'll get the idea.Hope you get it sorted out.good luck,non metal bodied cars are a nightmare for antennas.
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nikharr
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Re: Dipole theory adoption.

Post by nikharr »

Thanks George,

Yes I see what you mean now, I will try over the weekend, I will see if I can borrow an analizer before fitting the radio again.

Cheers Nik.

p.s. would dipole antenna wire be ok.
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Re: Dipole theory adoption.

Post by jazzsinger »

Hi Nik,

just about any old wire will do although insulated wire would probably be better (waterproof) unless you plan on running heavy power,in which case you would increase the diameter of the wire.be sure to waterproof all connections as once corrosion sets in it will soon travel the length of the cables via capillary action and cause all sorts of problems.

the final tuning would be trial and error on the 4 radial lengths,but 9ft should be there or thereabouts at 27 mhz.

for 27.78125 (ch 19 ukfm) being centre frequency,then the radials would ideally be around 8ft 5 inches.

the way to work it out is divide 300 by the centre frequency,in this case 27.78125 = 10.79metres,multiply that by the velocity factor of the wire,usually around .95 = 10.25m,divide by 4 for a quarter wave = 2.56m which is the size each radial should be at 27.78125 mhz,to convert to feet multiply it by 3.28 = 8.41 feet which is roughly 8ft 5 inches,add a little extra to each radial to allow for tuning.

that is exactly the same formula you would use to find out the length each side of a dipole too.for different bands just substitute the 27.78125 for whatever the centre frequency is of the band you want to cover.the diameter of the wire would very slightly affect the bandwidth,but the inherent bandwidth of the actual antenna itself is probably more critical.bottom line is it won't be a perfect install,but hopefully will be efficient enough to be useable over a decent bandwidth.

hope that helps you sort it out Nik and understand some of the theory behind it too. :D :D

had you used two loops which would have been approx 18 feet each they'd probably have presented a high impedance to rf at that frequency,not ideal and the reason base antennas use 1/4 wave radials which present low impedance.
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nikharr
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Re: Dipole theory adoption.

Post by nikharr »

Thanks again George,

The wire I have is brilliant, I used it for a 1/2 wave 40m dipole.
I used a calculator for the lengths, I took a lot of time and care to measure.
It went up and needed no trimming, I did allow extra wire folded back but didn't need it. It is perfect accross the 40m band with no extra work.

The use of radials for a ground plane is new to me, so your advice has been valuable and will be put to good use for the following week.

Cheers Nik.
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fez
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Re: Dipole theory adoption.

Post by fez »

jazzsinger wrote:
JamesA wrote:This any use?

http://tinyurl.com/yext8sb
they don't work,they coudn't possibly improve swr as they are either connected to the radiating element or connected to nothing at all,not to mention they are far too short to be efficient.definately snake oil.
Have to agree with that 100%, Don't understand why people use these?
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Re: Dipole theory adoption.

Post by JamesA »

No idea myself, not used one. Just came across it while browsing and looked like it could make some sort of ground plane.
M6JCA -> 2E0JCA

Yaesu FT-897, FT-857D (In car), FT-817D & Wouxun KG-UVD1P
Full wave 40M wire loop for HF, Watson V2000 for 2, 70 & 6
HRD Logbook: http://www.hrdlog.net/ViewLogbook.aspx?user=2E0JCA
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