Why use an Un-Un

[KL7AJ]

It seems there've been a lot of questions about "un-uns"
on QRZ of late. # Some folks wonder whether it's really necessary to use one on a properly designed ground-plane antenna, since the antenna is unbalanced anyway.

Yes it is! # But first, a little side note

It probably isn't too important the actual configuration of your un-un...whether it's a true un-un, or just a good r.f. current choke. #The important thing...the bottom line...is this...does it effectively reduce (or ideally ELIMINATE) current flow on the outside of the transmission line? #This is the end goal...to make your transmission line NOT radiate.

The only real way to know this is by actually MEASURING the current flowing on the outside of the transmission line. #I built a very effective and sensitive indicator for this, using a 24x36" wooden picture frame with a single turn copper wire stapled around its perimeter. #I come off of the loop with a short piece of coax to a full-wave germanium rectifier and a microammeter. #I mount the whole thing on a broomstick....sort of looks like an overgrown metal detector. #Looks funny, but it tells the gospel truth about transmission line currents (and antenna currents as well).

The results of this were surprising to me. #Of course, they were no surprise to W8JI et al, who have spoken eloquently on the subject. #But nothing drives home a point like actually MEASURING it for yourself.

I have a pretty "ideal" 40 meter vertical....I thought. #It's got four horizontal radials, about twenty feet off the ground. #I'd been using it for decades. #It worked fine, no complaints at all. #But just out of curiosity, prompted by one of JI's articles, I believe, I decided to build this current probe and measure the current in the radials and the transmission line. #Lo and behold, the transmission line current was about the same as the radial currents! #How could this be?

Interestingly, NEC-2 modeling showed the same results...at least if the transmission line was close to a multiple of a quarter wavelength...which is what my home situation was.

The good news is, a simple current choke comprised of a half dozen turns of coax just below the radial plane TOTALLY eliminated any measureable current! #This was even more surprising than the fact that my transmission line had such high currents to start with.

The bottom line is this. #MEASURE everything you can measure. #Theory is great...but it's even better when it actually WORKS!

73

eric

[KA4DPO]

You expect these guys to actually measure something?

Antenna current? What's that?

The new answer to a poor signal seems to be more mike gain and a bigger amplifier..

On a more serious note, I wish more new hams would take your advice, it would certainly save them a lot of hedaches.

[N2RJ]

Good advice, Eric.

I actually did put a feedline current choke on my vertical's cable. I never measured the currents but I guess I should do so with your proposed apparatus.

[G0GQK]

I'm a bit confused about this, un-un business.
Un-un means unbalanced to unbalanced, right ?

Now, if you have a 40 metre vertical with 4 counterpoise tuned radials the antenna is balanced, isn't it ? Its just a dipole with one half pointing upwards.
So, why do you need an un- un.?

Where an un-un is used is on a car, where the whole system is unbalanced. with a dipole a bal-un is used, balanced to unbalanced, the dipole is balanced and the co-ax is unbalanced,

As far as stopping RF running up the outside of the co-ax, putting 7-8 turns on the feeder has been standard practice since Adam was a lad !

G0GQK

[N4CD]

Putting an inline choke in some applications may solve the problem. On lower bands, you might need a lot of turns.

If you have an RFI situation, like for example in the cable between a VCR and TV set, sometimes putting in an isolator, to actually break the shield, will solve the problem of generating ground loops in the system. It is 'rf ground' but there is no direct connection either in the shield or in the center conductor. (you can do that with back to back 72 ohm to 300 ohm line units).

Same goes on ground currents - the choke is only so effective. Putting in a un-un might improve patterns or rfi in the shack. You might have a dc or a/c power line ground loop in the shack - and that 'ground' at the far end of the coax might be giving you problems.

Doesn't hurt to try it.