[Elecraft] KPA1500 Tuner Question for the Group

[David Gilbert]

Well, not all 10:1 conditions are equal (low impedances tend to give 
higher tuner voltages and power loss than higher impedances), and of 
course 160m is generally the most problematic band because it takes so 
much capacitance and inductance to get a decent result.  But playing 
around with TLW, the free transmission line software that comes with the 
ARRL Antenna Book is very informative since it also includes a built in 
tuner calculator.  For a typical high pass T-Network tuner, the internal 
voltages can approach 4,000 volts at 1.8 MHz with a 20 -j90 load 
(roughly 10:1 SWR), depending upon the size of the output load 
capacitor.  That pretty much rules out relay switch tuning unless anyone 
knows where they can steal a large quantity of vacuum relays, so we're 
talking some pretty large motor driven components if it was to be an 
auto tune unit.

I'm in the process of building my own high power manual tuner for use in 
the shack, and it will be a high pass T-network with additional high 
power ceramic doorknob capacitors that can be switch selected in 
parallel with the main breadslicers.  There are some incredibly high 
power NOS doorknobs available on eBay from Russian and Ukrainian vendors 
at remarkably low prices.  I've bought from several suppliers and have 
been pleased each time.  Here's an example from an Austrian vendor from 
whom I have recently purchased these exact units:

*https://tinyurl.com/ybofl2os

*In my case, in addition to switchable fixed parallel capacitors for 
both the input and output variable capacitors, I will also have a switch 
that shorts out one or the other in order to convert the tuner to an 
L-Network in case the load prefers it.

Many years ago somebody whose callsign I have since forgotten came up 
with the idea of making a remotely tuned high power L-Network tuner by 
coupling the shafts of a rotary inductor and a variable capacitor 
together and turning them at the same time with a single DC motor 
through a gear reducer.  It's not as precise as being able to tune each 
individually, but it doesn't really need to be.  The capacitor goes 
through it's full range every turn of the inductor, and you simply 
reverse the polarity of the DC voltage to tune back and forth until you 
see a match at the shack.  However, unless you have a REALLY large 
variable capacitor, or some means of switching in parallel capacitors, 
this technique probably doesn't work as well on the lower bands and 
you'd want to know in advance whether your load was higher or lower than 
50 ohms, which often determines whether you use the capacitor input or 
inductor input configuration.  It's a great way to make a wide range SWR 
extender for a single band, though.

Sorry ... this post turned into more of a ramble than I planned.

73,
Dave   AB7E




On 4/21/2020 7:59 AM, Lyn Norstad wrote:
> Since the tuner built in to the KPA1500 is not able to handle the full power
> of the amp at a 10:1 SWR (like the KAT/KPA500 Combo), what tuner(s) are you
> using if you need that kind of matching capability (i.e.: for balanced feed,
> etc.)?
>
>   
>
> 73
>
> Lyn, W0LEN
>
>   
>
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