770 mHz DIY coaxial dipole question

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dave3825

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I did not want to open an old thread. I was reading the following thread, http://forums.radioreference.com/build-your-own-antenna/259198-770-mhz-diy-coaxial-dipole.html , and was board so I made one. There are 2 unanswered questions in that thread. The first was why not use the braid instead of 4 radials soldered in place. The second was, was there any required distance between the end of the braid/radials and the feed line.

I made one and left about 4 inches between the connector and the end of the braid. I did not have any db increase, in fact, it was about -10db. This of course was compared to the rs 800 mhz antenna.


I did try a rubber duck mod for airband,http://forums.radioreference.com/build-your-own-antenna/261896-vhf-airband-mod-rs-uniden-ducks.html , and that yielded me a 11db increase. That was compared to using the duckie with and without the wire. But no luck with the 700.

I live in an apartment and can not do outside or roof installs which sucks at the moment. Has anyone made one for 700 and seen any improvement?

Thanks
 

dave3825

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My set up is as follows (in an apartment), in my window sill, a 1 port cable tv amp with a 90 degree elbow on the rf input that the ant connects to. Amp outputs to 16 feet of coax under carpet that runs to my desk. Usually, the signal is passed thru a 4 way splitter, then to 3 dongles and a scanner. For my tests, it was a direct line from the amp into the sdrsharp.
 

lmrtek

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the length would be more like 3.25 inches for the radiator and 3.25 for the sleeve

what you need to research is as sleeve dipole
 

br0adband

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I've made a few experimental coaxial dipoles but as hertzian pointed out in that other thread the feedline is the problem more often than not and it will result in hampered performance by nature of the design and there's really not much that can be done about it.

As he also pointed out in that thread, a 1/4 wave ground plane is incredibly simple to make from #12-14-16 gauge wire or cut pieces of a metal coat hanger and an SO-239 chassis mount along with a few nuts and screws to hold everything together, no soldering actually required. I build these things for experimenting pretty frequently and they always perform exactly as they should without any issues at all. It's pretty tough to not make one correctly whereas with the coaxial dipole if you have even one single strand of cabling or the shielding making contact somewhere you might not even be able to notice it'll kill the performance rather substantially.

The one I use cut for 772 MHz these days is this one:



Took me all of 10 minutes to make it from some coat hanger sections and that SO-239 chassis mount with nuts and screws but I use spade lugs to hold the coat hanger sections instead of trying to bend that stuff into an eye loop to screw down, just how I do things. And yes, it's indoors as all my antenna work has to be because of building limitations (just renting an apartment has such things sometimes).

The cabling I use with that (also seen running down the mast and held in place with some masking tape - it's cheap, yes, but it works) terminates in a BNC which attaches to a pigtail to MCX for one of my older R820T RTL-sticks. At 771.98125 (the primary CC frequency of the LVMPD simulcast system here in Las Vegas) the signal strength using a 29.7 dB gain in SDR# literally shows as -0.3 to -1.0 dBFS - if I switch to using the RS 800 antenna and tape it to the same mast in the same position the signal strength shows -9 to -12 dBFS so again, it works and it cost me like $3.50 for the SO-239 chassis mount and the rest of the parts I had literally laying around. :)

There are situations where the coaxial dipole proves useful, but I'd say for "casual monitoring" especially if you're limited to having an antenna of any kind inside a structure, even close to a window (I have a large 5' diagonal window about 3 feet from where this antenna is "mounted" on the mast beside my desk), a 1/4 wave ground plane will prove itself to be more useful. Considering the size for a 772 MHz cut 1/4 wave ground plane, it sure as hell can't hurt to make one and see what happens.
 
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