[TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

[Joe Subich, K4IK]

> From: Keith Dutson
> 
> Your SteppIR must have the complicated matching network to 
> work for all bands since element positioning is a compromise 
> with the design.  This network is lossy.

In fact, the SteppIR does not need a complicated and lossy 
matching network.  A matching network, other than a combined 
un-un/balun is not needed since the proper tuning naturally 
matches the antenna.  

> By contrast, a Force 12 multi-monoband has no such compromise 
> and no associated loss.

You have obviously not modeled any "multi-monoband" antenna 
and observed the significant tuning compromises that are 
necessary in order to achieve usable SWR on all bands.  I have 
looked at several commercial designs and by simply removing 
the elements for the "other" bands the antenna can be made to 
provide significantly more gain (at least one dB) or better 
front to rear rejection (at least 5 dB at the design frequency). 

Multi-monoband designs are the height of compromise where the 
infinitely adjustable antenna is, by it's very nature, capable 
of being optimally tuned at any operating frequency.  

So long as the elements are tuned properly for their position 
in the yagi, the absolute position is relatively uncritical.  
For a given boom length, so long as there are enough elements 
to maintain the required level of inter-element coupling, the 
gain does not vary significantly with the position of the 
"inner elements" (elements between reflector and last director).
Front to rear rejection and bandwidth will be effected ... but 
as long as the inner elements are not tightly "bunched" at either 
end of the boom, F/R is reasonably consistent (+/- 5 dB).   

SWR bandwidth is a function of the amount of coupling among 
elements and element tuning ... and in an infinitely adjustable 
antenna are not critical to the overall performance.  

> Give me the Force 12 antenna for simplicity, strength and 
> light weight.

You're welcome to your savings of 1/2 square foot and 10 pounds 
for the C3 over the 3 element SteppIR.  In return, you give up 
1 - 2 dB on 20/15/10, 3 dB on 17/12 (as well as a proper match), 
operation on 6 meters ... as well as good F/R on 20-12.  

73, 

   ... Joe, K4IK