Thanks for the info.
I would need to know how much resistance it has at 10 and 11 meters so that it will remove enough CMC.
8k/ohms or greater is what I need
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The "Ugly Balun"
- Thread starter Robb
- Start date
Thanks for the info.
I would need to know how much resistance it has at 10 and 11 meters so that it will remove enough CMC.
8k/ohms or greater is what I need
Hey Doc - looking at the bottom of the chart, you will see a legend for the colors. The highest amount of resistance to CMC flow -I'm guessing - occurs at the dark green banding. Maybe you or someone can shed some light on this if I'm wrong. Tuning the highest resistance to the frequecies being used - in this case it is 10/11 meters - is my reason for saying what I did. I figure on going with the 5 turns of RG-8 (same as RG-213 I guess) on a 4.25 inch coil form. Is this the correct way to go with it before I put it into action?
If your tearing stuff up like you say Robb then anything is worth a shot, especially if it is as cheap as just making a few coils out of your coax. I read somewhere that you need a minimum of 10x the antenna impedance for a choke to be effective, so for a 50 ohm antenna, you would need 500 ohms of impedance on the choke to be effective. That kind of holds true when you look at the chart as 500 ohms is the minimum he has bothered to plot.
Like in any potential divider, if the ratio is 10:1, then the current ratio is the same, 90% flowing into the antenna, 10% down the coax for a 500 ohm choke. More impedance on the choke is obviously better, but I wouldn't get caught up in perfection, just try to make the best choke you can for 10/11M and if it solves your problems then thats great, job done. If not then maybe back to the drawing board, lol.
Looking at the chart, your RG-8 shouldn't be very far away from the 213 results, so 5 turns on 4.25" should give you a high choking impedance.
One thing to mention though, RFI is not always caused by CMC's, it could be that your antenna is working perfectly well, or even too well for the other equipment in your house. Electronics with poor immunity, like computer speakers for eg. can be effected by a high field strength from a nearby antenna. If adding a choke doesn't help, you might need some ferrite rings adding to the speaker wire for your PC speakers, and some clamp on ferrites to you monitor cable and others. Add the "ugly balun" first though and see if you notice any difference.
Like in any potential divider, if the ratio is 10:1, then the current ratio is the same, 90% flowing into the antenna, 10% down the coax for a 500 ohm choke. More impedance on the choke is obviously better, but I wouldn't get caught up in perfection, just try to make the best choke you can for 10/11M and if it solves your problems then thats great, job done. If not then maybe back to the drawing board, lol.
Looking at the chart, your RG-8 shouldn't be very far away from the 213 results, so 5 turns on 4.25" should give you a high choking impedance.
One thing to mention though, RFI is not always caused by CMC's, it could be that your antenna is working perfectly well, or even too well for the other equipment in your house. Electronics with poor immunity, like computer speakers for eg. can be effected by a high field strength from a nearby antenna. If adding a choke doesn't help, you might need some ferrite rings adding to the speaker wire for your PC speakers, and some clamp on ferrites to you monitor cable and others. Add the "ugly balun" first though and see if you notice any difference.
In case you're wondering (or even if you're not
), that MFJ-915 is what is known as a W2DU balun. A lot of people make them, and they're very easy to make yourself if you have a source for the ferrite beads.
Robb, your story sounds like the old lady in the typical neighborhood that claims your CB radio broke her TV, washing machine, radio, vibrator, and telephone.
If your 100 watt rig broke your computer, which I seriously doubt, then call your home owner insurance and see if you can sue yourself, the radio maker, and Solarcon for damages.
LOL!
Truth is - my rig sets right next to my computer and monitor. Things happened to my computer since I hooked up the new 100 watt radio. First the monitor needed replacing and then the motherboard glitched at nearly the same time. I find that more than just coincidence - IMO. I build computers for others and work on them too for some time now. I made no claims on insurance - but I will put together a choke to ensure that the new computer parts aren't damaged. I'd rather be careful than another dollar shy...
B
BOOTY MONSTER
Guest
i wonder if 1 watt swinging to 60 or 80 could possibly have more harmonics or splattering outside of the intended frequency than something much more conserative like a one to four ratio would do ?
Robb, are you having these problems with the ground mounted vertical?
What kind of radial field do you have down?
PR
What kind of radial field do you have down?
PR
hmm dabbling in analogy
you buy a cat from the store, it says pussy on the box, your having some of that hmmm hmmmmm, but when you get it home expecting some fun IT humps your leg wags its tail and barks at the mailman,
sounds to me like you bought a dog,
http://eznec.com/misc/ibalun.txt
http://www.eznec.com/Amateur/Articles/Baluns.pdf
packrat,
a choke balun is not to stop stray rf from getting back into the shack, it is to stop the equal currents that flow inside the coax from been split between the radials and the coaxial outer shield,
cm current flowing on the coax outer shield or mast is current lost from the radials,
rob has the right idea on his imax with short radials, we have done the same thing and seen an improvement in signal and a big reduction in rf on the feedline even though the choke i wound had twice the coax we should have used.
a choke balun is not to stop stray rf from getting back into the shack, it is to stop the equal currents that flow inside the coax from been split between the radials and the coaxial outer shield,
cm current flowing on the coax outer shield or mast is current lost from the radials,
rob has the right idea on his imax with short radials, we have done the same thing and seen an improvement in signal and a big reduction in rf on the feedline even though the choke i wound had twice the coax we should have used.
I'd be both grateful and interested in hearing as many details as you can recall while putting the IMAX together with the ground planes and the coax choke. In particular, the distance of the coax choke to the antenna feedpoint for one. Type of coax used and if it was a continuous piece including the choke - as Marconi discussed earlier in this thread - would be another. What material you used to isolate the antenna from the mast - and so on...
There are so many people that buy and use these antennas; the info you provide could be helpful to quite a few members and readers!
rob, we had pics on here of one install but my local got pissed off with the forum and deleted eveything,
i have posted about what we did more than once on here in similar threads to this one,
on the imax i have seen people put the radials at the top of the bracket, i dont know where the best place to put them is,
the radial mounting hardware is cheap tat, a flimsy metal bracket but i have not seen one break so far, its usually the imax that snaps,
i saw an a99 not too far from me with 2 radial kits, one hanging down as usual and one pointing up, it made me ride around and stop for a second look.
i have posted about what we did more than once on here in similar threads to this one,
on the imax i have seen people put the radials at the top of the bracket, i dont know where the best place to put them is,
the radial mounting hardware is cheap tat, a flimsy metal bracket but i have not seen one break so far, its usually the imax that snaps,
i saw an a99 not too far from me with 2 radial kits, one hanging down as usual and one pointing up, it made me ride around and stop for a second look.