Do dual-band 2m/70cm antennas really work?

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Re: Do dual-band 2m/70cm antennas really work?

Postby G4RMT » Tue Jun 28, 2016 8:46 am

Many of the ones that have 3dB gain over a dipole tend to use a radiator that is 5/8th of a wavelength, with a loading coil to bring the impedance back down. Some of the ones I have seen the inside of have a different approach and use what appears to be a ⅝ or perhaps ¾ wavelength element at UHF sitting on top of another element that is shorter. A few coils to do the impedance conversion and the relationship between 145 and 430 (almost a direct 3rd) means they perform on both pretty well. I saw one broken one a few years back that appeared to have two ¼ wave dipoles on top of each other with a phasing harness. To be honest, how they get the UHF gain often quoted is a mystery. As you have to destroy them to look inside, quite what is in each one is best guess.

If you look at commercial antenna, you see more detail in their specs, and gain seems to be far easier to see it's origin. 6dB comes from 4 dipoles, 3dB from 2. 3dB from a base loaded 5/8th - two of these together giving 6dB. These when physical connected top to tail have some matching losses, so often get quoted at 5.2dB or so - not 6. Using two different types of radiator - a 5/8th over a ¼, for example produces a slightly odd polar pattern in the horizontal plane, so the gain figure gets distorted too - a ¼ wave has no gain, but topped with a 5/8th with it's lower angle of radiation seems to give something like 4.5dB.

I've noticed that few of the amateur dual bander seem to have any information about how they work, and I often wonder exactly what is inside.

My commercial single band UHF white stick has better performance than the amateur UHF white stick - BUT - the amateur one has decent VHF performance, the UHF only one does not.

That doesn't help much really as the real answer is unique to a single product. Diamond, for example, looks the same as the other Chinese ones - but inside? Who knows?
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Re: Do dual-band 2m/70cm antennas really work?

Postby gm21136 » Tue Aug 23, 2016 12:01 pm

Hi,
I read your post about collinears I use mine for scanning as well as transmitting on 2m and 70cm. On the subject of mounting antennas. Have you considered using ladders, I have a triple section ladder bracketed to the rear of my house, it goes to 29 foot, collapses to 11 foot and is easily raised or lowered. The last I read, ladders stored do not need planning permission Hi Hi., and they are reasonably cost effective, and stronger than most single poles, and of course multi use.

I hope this might address some of your problems

Best 73s

John
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Re: Do dual-band 2m/70cm antennas really work?

Postby G4RMT » Tue Aug 23, 2016 12:54 pm

I've not thought about ladders, but they're a rather neat idea. A long three section could easily be fixed to the house wall, and then the top section raised , then the bottom - pretty neat and simple. In my main work area, we use 16mm and 28mm spigots for attaching very heavy lighting equipment in TV studios and on location - these have loads of useful bracketry and would make sitting an aerial on the top an easy job. Image

This sort of thing - Never thought about using these as quick attach fittings for aerials?
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Re: Do dual-band 2m/70cm antennas really work?

Postby m0lsx » Tue Aug 23, 2016 2:06 pm

gm21136 wrote:Hi,
On the subject of mounting antennas. Have you considered using ladders, I have a triple section ladder bracketed to the rear of my house, it goes to 29 foot, collapses to 11 foot and is easily raised or lowered. The last I read, ladders stored do not need planning permission Hi Hi., and they are reasonably cost effective, and stronger than most single poles, and of course multi use.
John


I have used a step ladder as a portable support, but had never considered a ladder on the side of the house. My only concern would be that it could be used to gain entry to my home. Thieves have been known to smash their way in through a roof before.
A friend use to use a sailing boat in his yard to side step planning permission, as anything that needs guys can be counted as permanent, but a sailing boat mast got him above his bungalow & side stepped the heavy planning restrictions where he lives.
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