[TowerTalk] Inverted L SWR High

[TexasRF@aol.com]

Tom is right about swr, 2 to 1 vswr shows the losses are in line with what 
should be expected.

We all want a nice low vswr antenna though and there are many ways to make 
that happen. Way back in the olden days, before we knew about un-uns ,etc we 
made these corrections with an L network. The guys running Texas Bug Ketcher 
mobile antennas still do this and for the same reason. The L net work takes 
only two components, an inductor and a capacitor. The simple form places a 
capacitor across the feed point, value needed is that to transform series 
equivalent to parallel equivalent that has resistance= 50 ohms. The L network 
series inductance is free as it is made by extending the antenna length 
slightly to make it inductive enough to make  the series/parallel conversion 
reach R=50 ohms.

The case of vswr=2 is the easiest one to do as the series R=25 and the series 
L also=25 ohms. Conversion of these numbers to parallel equivalent gives R=50 
ohms and Parallel L=50 ohms. Therefore a capacitance of 50 ohms across the 
feedpoint cancels (resonates) the inductance and you are left with 50 ohms 
resistive and a vswr=1.

The Q of the L network in this case is q=x/r or q=1. The antenna will control 
the vswr bandwidth in this case as a q=1 contributes almost no bandwidth 
reduction in the system.

There are other solutions to the matching problem as well. A series C by 
shortening the antenna slightly and a shunt inductance will have the same 
results and an added benefit of high pass response to help (slightly) reduce 
AM Broadcast energy at the feedpoint. The un-un is no help here.

One other point here: The L network is continuously adjustable over a wide 
range as compared to 2:1, 4:1 9:1 etc steps of transformers.  Since the Q of 
the circuit is so low the capacitor can be made of a piece of RG213 but at 
1.8 Mhz a 50 ohm stub is pretty long and $$ as the length is 45 degrees (1/8 
wavelength) or about 45ft.

73 de Gerald, K5GW

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