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#1
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Balun/Unun help please.
Hi there,
I have a marine MF (1 to 7 Mhz) transmitter with only an antenna "screw terminal" for a long wire connection on the back of the set, rather than an SO239 or N type, and the set itself being grounded with copper sheet to a decent earth. My problem is, I need to feed an MF whip on the roof, some 50 feet away, through the building, offices etc, and dont really want the EMC and Health and Safety problems that would arise from 400W PEP happily radiating indoors with the recommended wire lead-in. So I'll need to use a feeder. I have decent low-loss coax (LMR type) but how do I couple it to the set? ..and to the whip (just a long-ish wire in fiberglass) at the other end. Im thinking an unbalanced to unbalanced transformer at both ends of the coax should do it, with both the tower and the set in the workshop grounded but the coax outer isolated. I have toroids that will do it, but how should I wind them? Im thinking MF ununs could be made by winding 10 or 12 turns of coax on suitable toroids. Is this reasonable or am I way off. ALL suggestions, even those calling me a muppet , will be very gratefully received. Thanks, John G0WPA. |
#2
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Oh also,
Should I use the choke unun at the whip or at the TX or one at both ends? Thanks again, John. |
#3
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You may be asking for help with a solution that isn't necessary, and
also may not be sufficient to accomplish what you want. If the transmitter is capable of matching a variety of loads, it may be sufficient to simply run the coax from the transmitter to the roof, connecting at the transmitter end to the screw terminal (to the center coax conductor) and the transmitter's ground/chassis (to the outer coax conductor). At the roof end, connect the center conductor to the whip, and the outer to a ground plane...at least some radial wires, if there is no large metallic plate to connect to. But how will you know if the transmitter has properly matched to the load presented by the coax+antenna? It's usual to have some sort of tuner (perhaps already in the transmitter?), and perhaps an antenna at the other end of the coax that has a reasonable feedpoint impedance. It could result in a fairly high SWR on the line and still be reasonable, so long as the transmitter can match it. But I don't see that an "un-un" would do you much good in this case, unless you need galvanic isolation between the transmitter and the antenna. -- It also may be convenient to mount a different antenna connector to the transmitter, either in the existing sheet metal or on a small bracket attached near the existing screw terminal. Cheers, Tom John - G0WPA wrote: Hi there, I have a marine MF (1 to 7 Mhz) transmitter with only an antenna "screw terminal" for a long wire connection on the back of the set, rather than an SO239 or N type, and the set itself being grounded with copper sheet to a decent earth. My problem is, I need to feed an MF whip on the roof, some 50 feet away, through the building, offices etc, and dont really want the EMC and Health and Safety problems that would arise from 400W PEP happily radiating indoors with the recommended wire lead-in. So I'll need to use a feeder. I have decent low-loss coax (LMR type) but how do I couple it to the set? ...and to the whip (just a long-ish wire in fiberglass) at the other end. Im thinking an unbalanced to unbalanced transformer at both ends of the coax should do it, with both the tower and the set in the workshop grounded but the coax outer isolated. I have toroids that will do it, but how should I wind them? Im thinking MF ununs could be made by winding 10 or 12 turns of coax on suitable toroids. Is this reasonable or am I way off. ALL suggestions, even those calling me a muppet , will be very gratefully received. Thanks, John G0WPA. |
#4
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#5
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John:
1 to 7 MHz is almost 3 octaves, and no simple antenna is going to present a usable impedance match to the coax over that range. Are you operating a spot frequency, a narrow band, or do you need the full range? If the latter, then you better look at remote antenna tuners, although I do not know of any which go below 1.6 MHz. Because, no matter what chokes, etc. you install, you will never decouple the outside of the coax from the antenna field enough to be able to successfully run it in a tray with low level signals. Been there, tried to do that. One thing you will eventually learn, forget the half-antenna (whip) and put up a dipole. Then, you have a fighting chance to keep the antenna currents in the antenna where you want them, and not on the outside of the feedline, which thinks it is the other half of the antenna when you employ a whip, even with radials. At 1 MHz, the radials need to be almost 300 feet long. -- Crazy George W5VPQ Remove N O and S P A M imbedded in return address "John - G0WPA" wrote in message ... Hi there, I have a marine MF (1 to 7 Mhz) transmitter with only an antenna "screw terminal" for a long wire connection on the back of the set, rather than an SO239 or N type, and the set itself being grounded with copper sheet to a decent earth. My problem is, I need to feed an MF whip on the roof, some 50 feet away, through the building, offices etc, and dont really want the EMC and Health and Safety problems that would arise from 400W PEP happily radiating indoors with the recommended wire lead-in. So I'll need to use a feeder. I have decent low-loss coax (LMR type) but how do I couple it to the set? ..and to the whip (just a long-ish wire in fiberglass) at the other end. Im thinking an unbalanced to unbalanced transformer at both ends of the coax should do it, with both the tower and the set in the workshop grounded but the coax outer isolated. I have toroids that will do it, but how should I wind them? Im thinking MF ununs could be made by winding 10 or 12 turns of coax on suitable toroids. Is this reasonable or am I way off. ALL suggestions, even those calling me a muppet , will be very gratefully received. Thanks, John G0WPA. |
#6
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It sounds like you have an older marine transceiver with built in
antenna coupler, which is not uncommon. A tech would install the radio and match the whip (non resonant) and random wire feeder by choosing coil taps and tweeking a cap for each channel. The channel selector will now select the correct coil/cap to match properly the antenna. If the radio or antenna is moved you must retune the network to match the antenna again. If this is the case here you would have to bypass the coupler section and remote it to make the radio happy feeding coaxial cable, probably a big project. Mike John - G0WPA wrote: Hi there, I have a marine MF (1 to 7 Mhz) transmitter with only an antenna "screw terminal" for a long wire connection on the back of the set, rather than an SO239 or N type, and the set itself being grounded with copper sheet to a decent earth. My problem is, I need to feed an MF whip on the roof, some 50 feet away, through the building, offices etc, and dont really want the EMC and Health and Safety problems that would arise from 400W PEP happily radiating indoors with the recommended wire lead-in. So I'll need to use a feeder. I have decent low-loss coax (LMR type) but how do I couple it to the set? ..and to the whip (just a long-ish wire in fiberglass) at the other end. Im thinking an unbalanced to unbalanced transformer at both ends of the coax should do it, with both the tower and the set in the workshop grounded but the coax outer isolated. I have toroids that will do it, but how should I wind them? Im thinking MF ununs could be made by winding 10 or 12 turns of coax on suitable toroids. Is this reasonable or am I way off. ALL suggestions, even those calling me a muppet , will be very gratefully received. Thanks, John G0WPA. |
#7
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Crazy George wrote:
John: 1 to 7 MHz is almost 3 octaves, and no simple antenna is going to present a usable impedance match to the coax over that range. Are you operating a spot frequency, a narrow band, or do you need the full range? If the latter, then you better look at remote antenna tuners, although I do not know of any which go below 1.6 MHz. Because, no matter what chokes, etc. you install, you will never decouple the outside of the coax from the antenna field enough to be able to successfully run it in a tray with low level signals. Been there, tried to do that. One thing you will eventually learn, forget the half-antenna (whip) and put up a dipole. Then, you have a fighting chance to keep the antenna currents in the antenna where you want them, and not on the outside of the feedline, which thinks it is the other half of the antenna when you employ a whip, even with radials. At 1 MHz, the radials need to be almost 300 feet long. -- All very good points. What it comes down to is that a whip always needs some kind of ground/radials/counterpoise connection at its base. You need something to feed the whip *against*. Older marine transmitters is that they tended to take a good 'ground' return for granted, so they only provided a single terminal for the antenna wire. If you convert to a coax output, it won't make any difference unless you have something to connect the shield of the coax to, out at the base of the whip. That "something" has to collect all the RF return current from the whip itself... and if it doesn't, all the building wiring and the outside of the coax will collect those currents instead. If you think of your office building as a very small 'ship', but without the benefit of all-metal construction or a decent salt-water ground, this is not a promising situation. In today's electronic office environment, I'm afraid you are likely to be fighting a perpetual losing battle against EMC problems. If this radio link matters, you'd do better to find another location for the transmitter and antenna. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
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