[Elecraft] [OT] High SWR on 20 and 40

[Don Wilhelm]

Dave,

No, you are not an idiot, nor drunk, but you may be confused by the 
impedance transformation effects of a feedline.
I suggest you take a look at the ARRL Handbook or Antenna Book chapter 
on Transmission Lines, especially the sections dealing with transmission 
line matching sections if you want an understanding of what is going on.

The most common feedline matching section used by radio amateurs is the 
quarter wavelength section - that one is relatively easy to understand 
and the math is not complex.  The underlying mechanism is that a low 
impedance at one end will transform to a high impedance at the other 
end.  For example, if a 50 ohm electrical quarter wave of coax is used 
and one end is connected to a 100 ohm load, the other end will have the 
equivalent of a 25 ohm load - and if the far end of that same coax is 
shorted, it will have a very high impedance (theoretically infinity, but 
practically >4000 ohms) at the near end

Any length of feedline (and any impedance feedline) that is not matched 
in its characteristic impedance at the load will act as an impedance 
transformer.

It is nice to know the resonance points for your antenna system, but 
what is really needed is the impedance (both resistive and reactive) at 
the desired frequencies of operation.  If we knew all your feedline 
parameters (type, velocity factor, precise length, etc) we could likely 
compute the impedance at any other frequency, but it is easier to just 
measure it since your MFJ259 provides you with a tool to do exactly that.

There is no impedance transformation on a transmission line (of any 
length) at the points where the antenna is matched to that line - that 
is the situation at your "resonance" points if you only looked at the 
SWR to determine resonance.  The actual resonance point of the antenna 
*system* is where the reactance goes to zero (or very low as indicated 
on the MFJ259) - then at the resonance points you would look at the 
resistive component - it is not likely to be 50 ohms.

One other piece of information that may help your 40 meter situation is 
based on the fact that you can use the tuner successfully at the balun 
location - and that is that an electrical half wavelength of 
transmission line will repeat the impedance at each end - so if you 
connect a total of a  half wavelength  transmission line between the 
tuner and the balun, the tuner will tune it.  A 40 meter electrical half 
wave of solid dielectric coax will be in the vicinity of 42 feet 
(VF+0.66, and the length of the foam dielectric coax will be close to 
55.5 feet (VF+0.84).  The lengths you have been trying are closer to a 
quarter wavelength.

73,
Don W3FPR

djmd wrote:
> I picked up an analyzer today (259B). It's my first one, and I hardly know
> what I'm doing, but I can tell you right off the bat that whether I attach
> it to the coax in the shack, or directly to the balun, it's appearing to be
> resonant at the same 3 places. Around 10mhz, 20mhz, and 26mhz. If there is
> any other information you would like to know from the analyzer at any
> specific frequency and connnection point, please let me know.
>
> I hope this helps... I sincerely appreciate everyone's assistance. I'm still
> learning and it's nice to have a place to get good info and not made to feel
> like I'm an idiot. 
> :drunk::confused:
>   
>