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Old December 9th 04, 11:12 PM
John - G0WPA
 
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Default 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.
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Old December 10th 04, 12:30 AM
John - G0WPA
 
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Default

Oh also,

Should I use the choke unun at the whip or at the TX or one at both ends?

Thanks again, John.
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Old December 10th 04, 12:40 AM
K7ITM
 
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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.


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Old December 10th 04, 01:59 AM
Richard Clark
 
Posts: n/a
Default

On 09 Dec 2004 22:12:30 GMT, RM (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.


Hi John,

If you are adventurous enough to open her up, replace the "screw
terminal" with an honest SO239 (or "N" or "BNC" or "RCA") bulkhead
connector. This will require you to probably open up the hole to
match connector dimensions and perhaps drill the bulkhead mating screw
holes. None of this is terribly hard.

Or simply get a SO239 (etc.) bulkhead connector and solder two wires
to it: one into the metal through one of the mounting holes, the other
to the conventional center conductor solder cup. Connect those two
wires to the set: the center conductor wire to the existing "screw
terminal" and the mounting hole wire to the chassis (add a sheet metal
self tapping screw nearby the "screw terminal", and make sure the
screw does not puncture anything on the other side of the chassis
face). Insulate exposed wiring with tape. Leads should be kept
short, less than 4 inches or so. Make sure these connections are
hardy, one lead open can take you right back to RF in the Office.

As to your other question, add a 1:1 Current BalUn (or UnUn if you
prefer) at the antenna end of your cable. Make sure it is adequate
for your operational frequencies.

73's
Richard Clark, KB7QHC
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Old December 10th 04, 05:32 AM
Crazy George
 
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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.





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Old December 10th 04, 08:12 AM
Mike
 
Posts: n/a
Default

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   Report Post  
Old December 10th 04, 09:30 AM
Ian White, G3SEK
 
Posts: n/a
Default

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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