Topband: 'Hairpin' matching

[Tom Boucher]

Just to add a comment on this thread: 

I do not use an inductor to match my inverted 'L', just a capacitor from coax centre/bottom of wire to the radial point. Further I do not use big wide space 'toast-rack' Cs, but tiny ceramic ones rated at 6.3KV which will handle very decent amounts of power and are readily available over here. They seem to handle the high currents at the bottom of the quarter wave 'L' quite happily.

I measure the impedance at resonance of the antenna without any matching network, then use 
www.sandiego.edu/~ekim/e194rfs01/jwmatcher2.html

to calculate the L-network values needed. Fit the parallel capacitor then lengthen the antenna slightly to bring it back to the required frequency. This provides the inductance necessary for the L-network. 



Needless to say, your MFJ, or Palstar ZM-30 in my case, antenna analyser will probably give you the wrong sign for the reactive part of the antenna's impedance. This had me going for a long time before I realised the problem! Check this by moving the analyser frequency LF from resonance and the impedance should show -jX (capacitive). Moving the analyser HF from resonance should show inductive reactance (+jX).



My inverted 'L' needs 1600pF to give me 50 + j0.



I have also made switched L-networks to successfully resonate the same antenna on some other bands.



73

Tom G3OLB