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