[Elecraft] [BULK] Antenna tuning or matching unit

[David Gilbert]

Agree with all of that.  I realized after I wrote it that I should have 
stressed that zeroing out the reactance at the antenna from the shack 
end of the feedline does NOT match it at the the feedpoint and that 
you'd still have SWR losses in the feedline and also in the matching 
network.  I was referring only to the reactance and should have made 
that more clear.

Also agree about the output impedance of the transmitter, but I was 
trying to simplify the discussion and didn't want to make it more 
obscure than it had already become.

73,
Dave   AB7E



On 2/20/2018 11:51 PM, Jim Brown wrote:
> On 2/20/2018 9:35 PM, David Gilbert wrote:
>> "Antennas are tuned with wire cutters or a hacksaw so the feed 
>> impedance is the desired value, ideally 50 ohm resistive or at least 
>> a match for the feeder concerned."
>>
>> I think we're getting a bit off track by lumping "tuning" and 
>> "matching" together as if they are the same effects.  They are not. 
>
> Yep.  And many antennas are better matched to other than 50 ohm 
> feedlines. My high dipoles are in the range of 85-90 ohms, so I use a 
> good RG11 to feed them. And those dipoles are for 80 and 40M, so their 
> lengths are tweaked so that, with stub matching in the shack, the Z 
> presented to the 87A (tube) power amps is within the range they are 
> happy feeding.
>
>> At least in my world, tuning refers to compensating for reactance at 
>> the feedpoint by some means that nets it out to zero.  In that 
>> respect, doing so with a network at the other end of the feedline 
>> accomplishes the exact same thing as using wire cutters or a hacksaw 
>> on the antenna itself. 
>
> Don't agree with that -- the match between the antenna and the 
> feedline determines the loss in the feedline. The tuning network 
> between the transmitter and the feedline does NOT change the loss in 
> the feedline, it simply gives the transmitter a load that it's happy 
> driving, and to which it can deliver maximum power.  AND -- we are NOT 
> matching the antenna Z to the TX output Z -- for most real 
> transmitters, their output Z is LESS than the load they are designed 
> to drive, and that output Z changes dynamically with signal level! 
> Back in the olden days, we young EEs learned about how do determine 
> this with "load lines."
>
>> They are not different.     As you say, the current and voltage 
>> distributions are not the same as if you cut the antenna to length, 
>> but the lack of feedpoint reactance is. 
>
> But with most antennas, zero feedpoint reactance can be achieved at 
> only one frequency -- it takes something like a SteppIR to move that 
> zero point around the band. I may resonate my 80M dipole at 3625, but 
> I almost never operate there. Rather, I'm mostly between 3500 and 
> 3600, and between 3650 and 3850.
>
> And then there are directional antennas like Yagis. :)
>
> 73, Jim K9YC
>
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