[Elecraft] [K1] Question about antenna tuners that I should not have to ask but do.

[Douglas Hagerman]

Hi Don.

Thanks for the comments!

There is a good article about all this in the June 2015 issue of QST. My problem is that I can’t use open line because I have to go 100 feet, threaded through apartment rooms, to get from the antenna to the radio. So it has to be coax, and as you say, the ideal situation would be to have a balanced signal at 50 ohms at the antenna, connected to the coax, which would not be acting as a transformer because of the good match at the antenna end, and then feeding into the tuner which would say “good, it’s 50 ohms of resistive impedance at this end!” and not do anything.

Or I could throw a piece of wire off the balcony and hope that the pot-smoking hippie downstairs doesn’t freak out when he sees it.  :-)

Doug, W0UHU.




> On 28Sep, 2015, at 8:38 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> 
> Sorry for the blank response.  Blame it on late night 'fumble fingers'.
> 
> While 'common logic' will say to match the antenna to the line impedance, that 'common logic' is flawed.
> 
> It certainly will be that a good common mode choke is all that is needed at the antenna feedpoint, and that is a 1:1 impedance transformation.
> Its purpose is to keep common mode currents off the feedline and to keep the real radiation confined to the antenna.  That aspect is unchanged no matter whether the feedline is 50 ohms or 600 ohms.
> 
> The feedline will act as an impedance transformer no matter what (unless the feedline is exactly matched to the antenna feedpoint impedance).
> Keep in mind that open wire transmission lines are low loss (that is important).
> 
> If you connect a 600 ohm open wire transmission line to a 'resonant' dipole having a nominal impedance between 50 and 75 ohms, that transmission line (even thought it has an SWR between 8 and 12) will have extremely low loss.  450 ohm ladder line and 300 ohm ladder line are similar, but will have higher loss than open wire line.
> 
> The real problem is to match the impedance of the shack end of the feedline to 50 ohms.  The transmission line will act as an impedance transformer, and depending on the length of the feedline and the frequency, the shack end may be vary between a very low impedance and a very high impedance.
> 
> In other words, attempting to use a 4:1 balun or a 1:1 balun (or any other ratio) at the antenna feedpoint may be an exercise in futility.  It all depends on the feedline impedance transformation at the frequency of interest.  And that depends on the feedline length and the frequency.
> 
> One would use a 4:1 balun to match coax to an antenna feedpoint impedance of 200 to 300 ohms (folded dipole) or to a properly tuned OCF antenna, but for all other conditions, all bets are off unless you know the antenna feedpoint impedance of the antenna at all frequencies where you want to use it.
> 
> A study of the ARRL Handbook or ARRL Antenna Book section on transmission lines should provide you with more information.
> 
> 73,
> Don W3FPR
> 
> 73,
> Don W3FPR
> 
> 
> On 9/28/2015 8:02 PM, Byron Servies wrote:
>> On Mon, Sep 28, 2015 at 4:37 PM, Douglas Hagerman
>> <douglas.hagerman at me.com> wrote:
>> 
>> With 450 or 300 ohm ladder line, a 4:1 would be more appropriate to match the feedline to the expected impedance of the K1 input.
>