off-the-shelf dipole & balun

g2000 · Post by **g2000** » Sun Apr 23, 2017 9:01 pm

Currently in the process of getting a new station together... I've got a 100W TX and a potential broadcast location sorted. I've built a raspberry pi, with 192khz soundcard that will go at my TX point to play our LibreTime stream and add RDS

Was doing some range testing the other week - range was ok, but the amount of buzzing and interference to devices in the top of the building would be problematic for those living there. Must be RF being emitted from coming back down the coax I guess? I have a SWR meter (Not sure how good it is!), but SWR was high, 1.8 or so.... (Using adjustable sandpiper dipole set correctly(-ish) for frequency...)... Did manage to get it to 1.5 in my garden about 3m up...

From what I've read, a balun will help to stop this RF coming back down the coax and radiating of it, and maybe reduce the RF issue in the building? All speakers buzzing, and electronics generally very whacky currently!!!!

All the antenna theory is certainly not my string point, so I'm wondering is there somewhere I can buy an off-the-shelf, tuned, dipole, with balun that I can just plug an to via a single SO239 connector from my TX - so I'm not having to worry about if i've built something incorrectly, and just the the show on the road... or air....

Cheers muchos....

nrgkits.nz · Post by **nrgkits.nz** » Mon Apr 24, 2017 12:18 am

You need to mount the antenna up as high as you can get it, and it needs to be at-least 3m above the highest point on the roof, not 3m above the ground. SWR should be no more than 1.5, preferably 1.3 and less.

Here's the old NRG info for creating a balun:
http://www.gareth.net.nz/nrgworkshop/ha ... aerial.htm

The buzzing and interference to electronics etc... is due to RF getting into everything, this shouldn't happen if your antenna is mounted well clear of anything up on a roof top or long pole.

g2000 · Post by **g2000** » Mon Apr 24, 2017 8:41 am

Thanks for this. Do prebuilt baluns exist, that I can just connect in-line with a connector at each end?

With my initial test, it was on. Pole out of an attic window, but not quite clear of everything (want to be in the door as little as possible!)

So hopefully once properly mounted, these RF issues should be reduced? Is the value essential, or will mounting high and clear be my main fix here?

Ta

Albert H · Post by **Albert H** » Mon Apr 24, 2017 10:38 pm

High and clear is essential if you're not going to waste your expensively generated RF and cause interference in the building (and nearby). Look at the diagram for a Pawsey Stub. It's simple to do, and you can add it to your dipole in about 5 minutes if you can solder. If you can't solder - learn, quick!

g2000 · Post by **g2000** » Wed Apr 26, 2017 7:11 pm

Right, going to have a bash at this then.

Questions on he pawsey stub length... I'm looking at 107.0mhz and use URM67 50ohm coax...

Some places say the velocity factor of this cable is 66%, others say 0.85 - can anyone offer any pointers on this?

Cheers muchos!!

g2000 · Post by **g2000** » Wed Apr 26, 2017 8:06 pm

Doing some more reading of http://www.aeroelectric.com/articles/BA ... Baluns.pdf - maybe velocity factor isn't that important...

So just a balun length of a quarter wavelength, so 66cm in my case of 107MHz?

Albert H · Post by **Albert H** » Thu Apr 27, 2017 12:31 am

Correct. 66cm should do the trick!

g2000 · Post by **g2000** » Mon May 01, 2017 1:33 pm

So, it didn't take quite 5 mins, but I did it! SWR reading in my garden on 3m pole was nearly at 1.2 - cheers for the advice.

Went to have another go sticking pole out of attic room window, but still got loads of buzzing through speakers on various devices in the room. Once properly roof mounted 3m clear of anything, will that issue stop/reduce? I do have quite a bit of excess coax, I guess that's a bad thing and should chop it as short as possible for it's real home?

Maximus · Post by **Maximus** » Mon May 01, 2017 2:00 pm

Yes the buzzing should stop. Whenever I use the antenna in the loft one of the radios buzz (whilst in standby) and the CFL lights glow. Even when the SWR is 1.5:1

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radionortheast · Post by **radionortheast** » Sat Jun 10, 2017 6:27 pm

i'd say leave the cable long, don't want to be cutting it too short, the other thing is changing lenght of co-ax can lead to mismatch