Next Question

[freecell]

"tune the antenna and you wont have to worry about coax length."

i can cite numerous examples where antennas have been installed and regardless of tuning adjustments an swr below 3:1 was not obtainable. this happens quite often where the input impedance of the antenna is as low as one third of the value required to match the antenna to the generator via a 50 ohm feedline. your methodology doesn't work in these instances.

yes bob, the impedance does change, anytime the load separate from the feedline measures anything but 50 ohms resistive and 0 ohms reactive.

 

[C2]

So, in that case, you are saying that 18 feet of coax will fix the problem?

 

[freecell]

"So, in that case, you are saying that 18 feet of coax will fix the problem?"

no, the 3:1 swr still exists using 18'.

 

[W5LZ]

freecell said,
"i can cite numerous examples where antennas have been installed and regardless of tuning adjustments an swr below 3:1 was not obtainable."
----
In that case I would have to assume that the antenna's resonance was what was being 'tuned'. The solution to that sort of problem is that after finding 'resonance' you start 'tuning' the antenna's input impedance. Using coax length and characteristic impedances to do that 'tuning' is only one way of doing it. Try one of the others. You might be suprised...
- 'Doc

 

[freecell]

the antenna has a tunable tip, that's it. the characterisitc impedance of the feedline is no help here as it no longer exists due to the 3:1 mismatch present at the antenna feedpoint. all you're concerned about is that the transmitter is able to deliver as much available power to the load as possible and you have two or three minutes to solve the problem. what do you do?

 

[W5LZ]

Ask them if they want it fixed, which might take little longer, or if they want it 'kloodged' which will only take a couple of minutes.
- 'Doc

 

[FL Native]

if i ran into that i would think I need more time, and its probly a coax problem or bad counterpoise. <<< location,or body of vehichle just isnt grounded very well>>>

 

[Captain Kilowatt]

Ask them if they want it fixed, which might take little longer, or if they want it 'kloodged' which will only take a couple of minutes.
- 'Doc

I take that to mean you would ask them "Do you want it done or done right?" That's what I would ask.

 

[freecell]

the goal is to restore as much of the 20 - 25% power lost at the transmitter as a result of the load mismatch in the shortest amount of time possible. the amount of the lost power that is restored will dictate just how "right" it's been done but we don't know that yet because no one has answered the question.

flnative, you're on the right track but there is no problem with the feedline, that is if you don't count the standing waves on the line. one or more available solutions require their presence to be effective and if you haven't figured it out yet counterpoise or ground mass is present but insufficient.

 

[W5LZ]

"counterpoise or ground mass is present but insufficient.'... good 'line', stick with it!
- 'Doc

 

[bob85]

are we talking a purely resistive load?
with a best match of 3:1 i guess you could use a series matching section of dissimilar characteristic impedance coaxial to get the match you need,
im sure it would not take you long to calculate what electrical line section lenghts you need knowing the load impedance and the main and matching section impedances and vf then with a calculator figure out what lengths of each would result in a good match to an antenna that is far from the original feedline characteristic impedance,
dont ask me to do it for ya but please show us how to calculate such a series section if this is the way you would do it.

 

[bob85]

the maths is not so simple, arctan i dont have it on my calculator :shock: so i found an online calculator and guessed how to do it while i wait for your reply on series matching even if this is not how you would do it,

using 27.205 mhz as the centre frequency,

7.12ft 107.4 degrees of 50 ohms coax in series with 3.25ft 48.99 degrees of 100 ohm coax both .66 vf to match a 17ohm purely resistive load to a 50 ohm transmitter, then you can feed this with any length of 50 ohm coax to get to the transmitter i think,
if 100 ohm is not available parallel 50 ohms could be used to make a 25ohm matching section then adjust the 50 ohm and 25 ohm lengths accordingly,

im half asleep but i had to have a guess once i realised you had posed a question

 

[freecell]

FOD

good job bob........that's one possible solution.

 

[FL Native]

see hes just trying to to teach us to fish its all good.

 

[bob85]

the only other way i can think of would be the dollar special idea i asked about further up the thread or maybe with a wire to ground or from the feedpoint to coax outer shield just the correct length for resonance and a good match to 50 ohm coax.