[Antennas] Re: question on short antenna diameters

Sat Feb 4 02:44:26 EST 2006

Discussing...
 "Antenna is a short center loaded vertical for 40m,
 mounted in the clear, consisting of a:
 3'x3' base plate with (3) 1/4 wave radials
 30" mast/stalk
 6"x6" coil
 60"-102" stinger
snip
 No. The diameters you mentioned have little effect on anything.
 The ground losses under your antenna, assuming the radials are at
ground
 level, dominate everything else. Your short antenna has a radiation
 resistance of around 5 ohms. The effective ground resistance of three
 1/4 wave radials is about 25 ohms. So dividing 5 by 25 and multiplying
 by 100 calculates to an over all antenna system efficiency of 20%. Any
 concern of conductor diameters pales into insignificance compared to
 your other losses. When you run 100 watts into that antenna, only 20
 watts is radiated.
Well let me add a little more to the mix here,
the 3'x3' plate with the radials attached...
will be placed on a 2nd story flat roof...
does this effect your comments any?"
Sam, the trend is toward increased efficiency, but it will be greatly
moderated by coupling into everything within the building (wiring,
plumbing, etc.). Also, I listed a best case senario above, ignoring
inductor losses and using a rather high value of radiation resistance so
as not to discourage you too much. So, up on the roof top efficiency
will still be on order of 20%. That is not a whole bunch of dB's though.
As for the short amount of 'fat' tubing increasing bandwidth, it would
be an amount measureable in a laboratory. Bandwidth is primarily set and
fixed by the high Q of the loading inductor.
73, Roy K6XK Iowa