I'm moving my feed wire in the attic further from my audio chain. I'm going to drop it down through the ceiling about 10' - 15' closer to the antenna, with the transmitter closer to the ceiling. I'm going to wind up losing almost 20' of cable. This might help my coverage problem too. I was going to run it through some metal conduit to shield the RF radiation, but I've heard that some types of PVC might accomplish the same result. Any thoughts on this idea? Like will it possibly cause more problems than it will solve? Thanks- PS-no, I haven't scored an SWR meter yet.
Theoretically your coax should not need shielding as, by nature, the rf travels on the inside of the cable (if it's terminated properly).
^-this.
If you are radiating enough power out of the coax shielding to be picked up by a field strength meter (or cause you problems in the studio) then there is something wrong with the antenna. The coax should not leak enough RF to be of any issue if it's load (antenna) is working properly.
If the antenna is working correctly but part of your cable is radiating power then you need to create a simple balun to trap any RF from radiating along your coax line. Taking 5 or 6 turns of coax about 6" diameter near the antenna will help trap out RF from traveling along the coax. With some types of antennas like dipoles, jpoles and so on it is required.
Obviously if the transmitting antenna is too close to the studio you can get RF back into your audio causing hum and other problems. You can use RF chokes, ferrite beads, or cheap snap on torroid cores to block any RF from getting into your mixer and audio equipment. Be lucky you are working with FM. Working with AM at high power near the studio is much more of a nightmare! I have heard of professionals installing .1uf caps on damn near everything to try and kill RF
By the way PVC will do nothing to shield your coax cable as far as leaking RF energy. Even metal pipe would be useless unless grounded really well and that isn't necessary in the first place if everything is working properly.
Thanks, that is the type of information I'm looking for. However, I shared this information with my RF go-to guy, the one that builds transmitters and stuff. He said that actually the coax is much like any other shielded cable, like mic cable for instance. The center is the "Hot" and the shield is the "Return", and both parts carry a portion of the signal. He constantly has problems with spurious emission from cables, even though he has grounded transmitters, etc. The PVC question is because I know that if you try to run an AM transmitter wire inside certain types of PVC it won't radiate. Apparently sticking it in a microwave is the test. I have a 15 watt with a BNC output. I'm using "Super premium" RG 6. It says that it's tested out to 300g, whatever that means. It runs about 40' from there to the antenna. The antenna is a quarter plane mounted on an 8' mast attached to a chimney. The coax enters the attic right above the transmitter. The transmitter is about 6' from the audio equipment. The coax is aluminum shielded, not copper braid. Yeah, I know-copper WOULD be better. Maybe it would be better if the transmitter, or antenna mast, were grounded. However, I'm on the third floor and a wire run down to the "physical' ground would probably act as a radiator on it's own. I don't want to use the 'ground' pin on the AC outlet because, although it should represent "earth", in a building this old that's not a guarantee. I have tried grounding the transmitter to the heating system, which makes a great ground for my AM transmitter, but it made no difference. I do know that when the transmitter is on it deflects my monitor VU meters slightly, and the signal overpowers all of the other receivers in the apartment, but not anywhere else in the building. If I move the wire around it makes a difference. So- should I work on better grounding, or simply move the transmitter further from the audio chain? I had planed on coming down from the attic several feet closer to the antenna, and mounting the transmitter closer to the ceiling. Whadaya think?
Darklife's tip about the balun is a good idea for vertical antennas that don't have an ideal rf ground (such as a groundplane). If the rf ground is not "ideal", the feedline will radiate to some extent because the feedline inevitably becomes part of the antenna.
I'd use about two turns at 6" diameter. That should be enough impedance to choke off feedline current that does travel on the outside of the coax. I'd also locate the balun at 1/4 wavelength from the feedpoint of the antenna. That distance is easily calculated if the transmitted frequency is known. Moving the balun a 1/4 wave away is good because it gets the balun further away from the high current that is at the feedpoint.
Good advice. I'll try it as soon as I can pay the guy the landlord allows on the roof. He won't let me go up because I'm too old and the roof is too steeply sloped. It's about $25 each visit. In the meantime I'll go ahead with moving the wire and relocating the transmitter. I wish I could find a good FM SWR meter on eBay that doesn't cost an arm and a leg. That would be a BIG help.
Update. I've been off the air for a couple of days. First of all I like to takes breaks every now and then just in case "they" are listening. And I have a 'break' in a bone in my foot, which has keep me hopping- :-). PLUS, a dedicated listener and close friend donated a Mac tower to replace my poor little laptop as a server. My laptop has been a trouper, but just wasn't built for running 8 hours a day. The Mac isn't up and running yet. I need him to come over and load my music files and run through the operation. ANYWAY- What I wanted to add is that I tried putting a balun at the transmitter output. I put 4 turns in the cable. I know it's not the best case scenario, but it DID make a difference. The meters on my monitor didn't move and the signal is as clean as, well, Mr. Clean! Also the buzz in my monitor amp and computer speakers across the room is gone. So it definitely did something. Many thanks for all of the comments. Maybe later on- when the weather gets better, I'll move the balun to the roof.
Good job on the balun. I'll bet you're happy to have clean audio and proper meter operation again!
I made a two-turn balun about 4" in diameter at the input of my swr meter (that balun is wound of the jumper cable that connects the transmitter to the input of the swr meter).
That coaxial cable jumper from the transmitter to the swr meter is the only unbalanced line in my system. The tuner and the line to the antenna are balanced. The only reason I wound that jumper into a balun coil is that before I had the line coiled, I noticed that if I put my hand around the coax, I saw the reflected reading change on the meter. I reasoned that I must have a small amount of current flowing on the outside of the coax to make the meter change.
I never had any audio difficulties with the balanced system, but figured it wouldn't hurt to make a coil to ward off any potential problems. The coil did stop the effect of changing the swr when I grasped the cable.
You could leave your present balun at the TX output, even when you add one at the antenna. It couldn't hurt.
Post by HighMountainRadio on May 18, 2018 3:09:15 GMT -6
Hey Brother !
The use of a 'RF Choke' consisting on 6 turn, 6" in diameter works great and is CHEAP ! Another good alternative is to utilize 'Ferrite Sleeves' of the 'MIX 43' type that have a hole large enough for RG-9913, RG-8/U, etc., .505" inside diameter ferrite sleeves aren't cheap from most sources, however, if you choose this splendid method, I would highly recommend purchasing them from Mouser Electronics, they are CHEAP priced, from them !! You simply slide your coaxial cable through 5 or 6 of these stacked inline. They should be placed at the antenna feed point. They work splendidly also but it is imperative to use the correct 'mix' of ferrite, in the case of FM broadcast, that mix is MIX 43. I'll update this later to provide you with the exact part number from Mouser..
I agree, PVC has NO RF shielding qualities whatsoever. Anyone who is serious about CORRECTLY installing any type of transmitting antenna should ALWAYS use a RF choke at the antenna's feed point ! It will prevent your coaxial cable's outer shield from radiating any RF which causes high reflected power and RFI or radio frequency interference and can easily screw up your audio at your mixer, etc..
I will append this post later today to show instructions and sources for ferrite sleeves which are used for the same purpose, some folks rather use these for convenience, either method is 100% effective !!
73... Spooky...
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
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