Elusive antenna...

Thread Starter

Externet

Joined Nov 29, 2005
1,655
Hi.
Cannot find it, perhaps does not exist/cannot exist and am chasing ghosts.

Do you know of any omnidirectional antenna for broadbandwidth reception of 470MHz to 700MHz ?

Does not exist? ----> How to make one, or a parallel? set/array of antennas to cover that spectrum ? Gain is very welcome, the more the better, no matter if has to be large.
 

ZCochran98

Joined Jul 24, 2018
166
No guarantee, but you may try looking at bowtie antennas or other similar lambda/2 bipolar antennas. The wavelength of the central frequency for that range is 523 mm (so a lambda/2 antenna will be, at most, 261.5 mm). Bowtie antennas are ok, and you can somewhat tune their bandwidth, but they're a little tricky to properly make and tune. Combined with an RF amplifier or a grid of them, you might get decent results.
 

Papabravo

Joined Feb 24, 2006
16,809
Hi.
Cannot find it, perhaps does not exist/cannot exist and am chasing ghosts.

Do you know of any omnidirectional antenna for broadbandwidth reception of 470MHz to 700MHz ?

Does not exist? ----> How to make one, or a parallel? set/array of antennas to cover that spectrum ? Gain is very welcome, the more the better, no matter if has to be large.
Omnidirectional is no problem. There are multiband antennas for VHF work. I have one that covers the 2M, 220 MHz., 440 MHz. amateur bands. It has a loading coil in the approximate middle to make it appear shorter at higher frequencies. The maximum length would be a quarter wave length of 470 MHz. or 0.15957 meters. The minimum length would be a quarter wavelength at 700 MHz or .10714 meters. For starters I would take the geometric mean of the two frequencies, that would be 573.58 MHz for a quarter wave length of 0.13075 meters. At that point on the 0.15957 meter quarter wave antenna I would form a 3-5 turn air coil. That's my best guess but you'd you'd have to try it and sweep it on a VNA to be sure.
 

SamR

Joined Mar 19, 2019
3,491
I assume from the wall wart that it is an active antenna.
1617675991215.png
Or a discone antenna... which has more bandwidth...1617676126930.png
 

Thread Starter

Externet

Joined Nov 29, 2005
1,655
-images borrowed from the web-


If this is a two-array of bowties :


1617677191592.png

removing the reflecting mesh am still unsure it will be omnidirectional. Should behave as dipoles.
1617677829338.png



If this dual bowtie array is rotated (mast horizontal ! and mesh removed, am more convinced it will work omnidirectional :
Am I far wrong ?

1617677885215.png
 

Thread Starter

Externet

Joined Nov 29, 2005
1,655
Thanks
Delta prime. Seems a too wimpy 6 - 8 inch, little gain and no gain-to-frequency plot disclosed. They use the brochure to say it is 'special' instead of important data.

SamR: That active one is similar to the one am using and it is not working well enough. :(
 

SamR

Joined Mar 19, 2019
3,491
I always thought bowties were semi-directional. Most mass-marketed active TV antennas that I've come across were more gimmick than a reality.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,655
That is my perception and literature found... bowties are more like dipoles than anything else... UNTIL flipped sideways. Then they behave omniazimuthal which is really what am after. Picking signals from all horizons around. "Omnidirectional" is misleading as would imply receiving from above/bottom equally well.

I was looking at that Betso antenna, fits the spectrum, seems a ripoff for such a little gain. No plotting of gain vs. frequency disclosed on the specs. :( Calculations seen for other designs achieve up to 7 dB, not 1.8dB
1617686215884.png
 

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SamR

Joined Mar 19, 2019
3,491
I live 90 miles from the closest TV tower and gave up on antennas when cable became available. No hassles erecting and maintaining a 50-70' Rohn tower with antenna and rotor which is what was needed in this part of the boondocks. But... Cable has gotten into disputes with "local" TV stations over rebroadcasting their signal and have been dropped rather than pay royalties to the local stations. So I've been looking at the new HDTV antennas which have much better specs than the better yagis of 40 years ago. Most come with a rotor but it goes full rotation before reversing, unlike the older rotor controller where you twisted the indicator to the direction you wanted to point the antenna at. So I've been considering mounting one on the peak of my eaves fascia @ 20-25' with a bracket to hold a short mast and REAL rotor. So far just daydreaming and specing it out in my head. Omnidirectional are good for "in town" say less then 20 miles from the towers but once you get over 50 miles you really need to look at a hi-gain directional antenna. Like this...
I am assuming since you said 470MHz to 700MHz that this for TV?
1617687892823.png
 
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Tesla23

Joined May 10, 2009
481
Let's look at the basic physics. If you want an omnidirectional horizontal pattern from a simple antenna element, then you won't do much better than a dipole at about 2dBi. The only way you get more gain is to increase your directivity - which means concentrating the energy in some directions at the expense of others (thinking as a transmit antenna). To do this whilst keeping your omnidirectional horizontal patters means that you have to squish the pattern vertically - change your donut into a pancake. You don't really want any gain significant;y above the horizon anyway, I imagine. The way you typically do this is to stack elements one above the other, for example, a collinear. The challenge is to get your bandwidth, you would need to stack a broadband element, like a discone (or a bow-tie).

Of course, I'm making assumptions about your requirements - a simple answer is a BIG biconical antenna - narrow flare angle flaring out to a vertical aperture several wavelengths high. But i guess you want to mount it on a mast and the wind loading would kill you.

What polarisation do you want - all these are vertical. Omni horizontals are more difficult / interesting.

If it's a land-based application receiving signals from a distance, often the easiest way to get 6dB more signal is to double the height.
 
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