Crazy 800MHz base antenna idea

[LIScanner101]

I just had a crazy idea…nothing new for me.

Suppose I were to buy (8) 800MHz Yagis, and arrange them in a circle, and connect the output of each of these antennas to a multicoupler and then fed the output to a single scanner, and put this contraption either on a rooftop pole or in my attic.

Assuming cost wasn’t an issue, would this result in the ultimate 800MHz trunking antenna? Or would it result in super overload?

Just throwing this out there….

 

[n5ims]

You'd probably get poor results. Unlike what you picture - the antenna with the strongest signal would feed the scanner and the others would feed nothing - you'll end up mixing in phase and out of phase signals together so the scanner could actually end up with no signal (most likely some signal but less than what a single yagi would provide). You also could easily end up creating your own version of the dreaded simulcast distortion problem that plague many scanner listeners due to multiple transmitters reaching the scanner out of phase.

 

[LIScanner101]

You'd probably get poor results. Unlike what you picture - the antenna with the strongest signal would feed the scanner and the others would feed nothing - you'll end up mixing in phase and out of phase signals together so the scanner could actually end up with no signal (most likely some signal but less than what a single yagi would provide). You also could easily end up creating your own version of the dreaded simulcast distortion problem that plague many scanner listeners due to multiple transmitters reaching the scanner out of phase.

You hit the nail on the head - I envisioned something that apparently doesn't jibe with reality. Thanks for setting me straight on this, much appreciated.

 

[ramal121]

I love crazy ideas, especially with antennas. Experiment, most of the time it's disappointing, but when it does work it's a real thrill.

Now back to your SWAG. I'll agree with n5ims that you'll be pumping different phase signals into a common feedline. At 800MHz this is easy to do as a wave length is only several inches.

Also if you combine several antennas together, the total parallel impedance is going to be quite low and the mismatch will affect the ability to transfer power to the receiver. You could use a passive combiner (kinda like a splitter in reverse), but the loss per port will negate any gain of the yagi antenna. You could put a pre-amp in, but it will have to be on the antenna side of the splitter which would mean a separate one for each yagi. Now this thing is turning into a monster and you still have the issue of phase cancellation from the different antennas.

I like how you think. If you come up with anything else whacky, post it

EDIT: what if you found a receiver that had an eight port diversity input that would automatically pick the best antenna for that transmission? Then your scheme would certainly work. Uniden? GRE? are you listening? Well probably not before I take a dirt nap...

 

[zz0468]

...what if you found a receiver that had an eight port diversity input that would automatically pick the best antenna for that transmission? Then your scheme would certainly work.

Not entirely far fetched. EF Johnson was doing diversity reception on their LTR systems and mobiles as far back as the early 80's. A variation of that could be accomplished with receiver voting, but there are some limits as to how practical that would be.

 

[Arizona_Scanner]

Not entirely far fetched. EF Johnson was doing diversity reception on their LTR systems and mobiles as far back as the early 80's. A variation of that could be accomplished with receiver voting, but there are some limits as to how practical that would be.

I've often wished there was a scanner with multiple antenna inputs, and a place in the software to tell a given system or frequency which antenna input to use. It would be an ideal solution for a scanner geek, though in today's market probably not very practical.

 

[NDRADIONUT]

Get a big fiberglass 800 antenna with omni gain.....

 

[K9DAK]

QST April Fool Antenna

Ha, reading this thread reminded my of an antenna for HTs I saw in an April issue of QST, somewhere between '95 and '07. It was a "spiral" antenna, starting with about a 1" diameter, with about seven coils, each increasing in diameter up to about 4", the turns spaced about 1" apart. I've searched through the QST archives and cannot find it again . . . anyone remember it?

Anyway, I built one as a joke for the ham friends I used to have lunch with at work every Wednesday . . . and we were amazed at how well the "joke" antenna actually worked!

So, I guess the moral of the story is . . . let your imagination run wild and try stuff, it just may work! And if it doesn't, oh well, try again. An old R&D boss told me, if you never have any failures, then you're not pushing the envelope of creativity hard enough.

 

[t0xPro-197]

hmm i like the idea of haven a 800mhz setup thats covering all side i gotta invest ina new outdorr but im mainly looking for one that enhances 800 mhz and 700mhz cause id like too have my pro-197 do nothing but thrunking i may just try building one ive got an old magnet mount with the coil in the middle of it just gotta look aorund but im reaily liken the one yall talking about its not omnidirc forgive the mis spell but its the price im on disablty i wounder if they are any cheap ones if were besides ebay dunno just talking outta my head ...if youall know of any good ant. building site please let me know and thanks

 

[zz0468]

I've often wished there was a scanner with multiple antenna inputs, and a place in the software to tell a given system or frequency which antenna input to use. It would be an ideal solution for a scanner geek, though in today's market probably not very practical.

Interesting idea... It's already being done in some ham gear that has multiple antenna ports. You can direct specific bands out through specific ports. It would add a great deal to the cost, and since scanner owners are notoriously cheap, probably not worth the effort. It might be easier to implement externally with PC control of scanners. Use the PC to direct an external antenna switch.

 

[e737]

Get a big fiberglass 800 antenna with omni gain.....

Have any recommendations?

 

[prcguy]

The idea in the original post would work but the overall gain would be moderate. For example if you started with eight 10dB gain Yagi's with equal length coax to an 8-way power divider you would easily fill in nulls from the individual antenna patterns.

In any given direction you would probably have two to three active Yagi's with potentially 3 to 4dB more gain than a single Yagi to give maybe 13-14dB gain just for an example. Some of that gets lost in the power divider with a net gain (difficult to predict) but available in all directions.

You would probably be better off with smaller Yagi's with patterns that would allow only four to give reasonable 360deg coverage, then a 4-way power divider or coax phasing harness would have about 3dB less loss combining them compared to an 8-way.

A 4-bay exposed dipole array is a good example where each dipole held out from the mast is a 2 element Yagi with some gain and when the four dipoles are arranged around the mast you have about 6dBD omni gain after the combining losses.

You would have very predictable results building two or four 4-bay dipole arrays and stacking them vertically. One 4-bay array would give about 6dBD omni gain, two arrays about 9dB and four arrays about 12dB omni gain.
prcguy