Topband: Re: Topband: matching an inverted-L

[i4jmy@iol.it]

Hi Bruce,

let's take it from the beginning with no hope to explain everything and 
knowing that it won't come out so perfect...

An antenna at feed point has its impedance, unless special cases this 
impedance is not a simple number but a complex one. 
Impedance can be purely resistive or instead reactive.
When the antenna is resonant the impedance is resistive and can be low 
as in a quarterwave vertical or high as in an half wave one.
An antenna like an inverted L is generally not resonant but longer than 
a quarterwave. Because of this lenght its impedance is not simply 
resistive but also reactive.
Since an antenna that's between a quarter wave and an half wave is 
inductive, with a device that has an opposite reactance placed in 
series to the radiator it's possible to cancel the reactance.
If an inverted L has an impedance of 60 +J188 (just an example) at feed 
point, we can't connect it straight to the 50Ohm line. We shall have a 
big mismatch and an high SWR if we do just. 
We must match somehow, and in this case is good and convenient to 
cancel that +j188 with a -j188 device placed in series.
The "device" is a capacitor whose reactance is capacitve, a -j188 at 
the frequency in use, more or less would mean 470pF at 1.8Mhz if I'm 
not making mistakes in mind calculations.
In the case the antenna feed point impedance would have instead an 
opposite sign, as it happens for example when the antenna is shorter 
than a quarterwave (capacitive antenna) the needed device to match 
can't be a capacitor but has to be an inductor, here it's explained 
your other question.
Things look apparently so simple, but often thay are more complicated. 
This technique only works (is applicable) when the resistive part of 
the antenna impedance is already enough similar to the line impedance.
In other words, if the invL is a 60 +j188 and we cancel that +j188 with 
a 470pf capacitor we shall have a 60Ohm load that show a good match, 
about 1:1.2 SWR that's a good one. 
If the invL impedance 
e a 90 +j188, only canceling the reactance 
we wouldn't obtain just a good match and SWR will be about 1:2.
Not all the invL can be matched so well with a simple capacitor. 
Coming back to the question about the capacitor isolation, if the 
antenna impedance is low then also the voltage across the capacitor 
won't be so big but of course it has to be proportioned to the used 
power. 
At 50 ohm 1500W are about 275V rms (388V peak) on a resistive load but 
across the capacitor things are different and voltage will be higher. 
Practically speaking I'll first check what's the input impedance of the 
inverted L and if I find the capacitor match method is applicable then 
I'd go for one that's at least 3KV isolated, possibly choosing a vacuum 
type that's inherently a weatherproof device.

73,
Mauri I4JMY
  
PS ..."tune" always for minimum SWR and minimum smoke.  


> Hi Steve and all ....
> 
> Be gentle with me as I am not a "builder." Help me understand this.