[Antennas] Re: question on short antenna diameters

Sat Feb 4 16:06:08 EST 2006

The impedance bandwidth of a driven element is the range of frequencies
above
 and below the center design frequency of the antenna that the driven
element's
 feedpoint will accept maximum power (rf), from the feedline. The design
goal
 is to have the reactance at the center design frequency of the yagi =
(0),,, (j +- 0).
 The impedance matching device will now operate at it's optimum bandwidth.
Wide
 element spacing, large element diameter, wide pattern bandwidth, and low
"Q"
 matching systems will all add to a wider impedance bandwidth.
----- Original Message ----- 
From: "Roy Koeppe" <royanjoy at ncn.net>
To: "Antennas reflector" <antennas at mailman.qth.net>
Sent: Saturday, February 04, 2006 2:44 AM
Subject: [Antennas] Re: question on short antenna diameters
> 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
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