[Elecraft] KX3 tuner loss?

[Ron D'Eau Claire]

You ask a commonly asked question. And, as usual, no responses so far. 

I can't give you an estimate either because first, the losses are highly
dependent upon the impedance transformation. You'd need to characterize that
doublet and feed line (the length of the feed line is important) to see what
impedance it presents to the ATU.

Then you need to measure the ohmic losses in the ATU inductors, wiring and
connectors. Dielectric losses may occur but with modern gear they are very
tiny at HF. 

Finally you calculate the currents flowing in the ATU at each impedance
transformation required to achieve a match to your feed line so you can
calculate the losses based on the resistance of the wires, inductors, and
connectors. 

Personally, I accept the idea that an L-network (which all the Elecraft ATUs
use) is highly efficient and leave it at that. 

If I were really inclined to actually measure the loss, I'd first measure
the RF power output into a stable 50 ohm load without the ATU in the
circuit. Measure with an external wattmeter at the load as well as the KX3
output power indicated.  

Then I'd breadboard another L-network with a variable cap and variable
inductor, connect it to the 50 ohm load and to the KX3 output, transmit and
adjust the external L-net (not the KX3 ATU) for a 1:1 match. In that way you
have created a known dummy load that presents the same complex impedance to
the KX3 as your antenna system. 

Now you can transmit at the same power at the KX3 as before and see what the
power is at the 50 ohm load. Actually, you will be seeing what the power is
after passing through both matching networks. 

My guess is that you'd find the total loss so low you won't be able to read
the difference on the two power meters. 

A more direct approach would be to isolate the ATU in a very well insulated
enclosure, transmit, and measure the temperature rise until it stabilizes.
You can then calculate how much energy is being dissipated by the ATU and
becoming heat. That requires the ATU be carefully isolated from the rest of
the KX3 and be in a very well insulated enclosure to avoid loss of heat to
the environment, something not practical to do with a KX3. It is, however, a
valid way to determine losses in external devices like an RF transformer
(e.g. "balun").

I suspect this explains why few (if any) Hams bother to make the
measurements.

73, Ron AC7AC 



 

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Barry
N1EU
Sent: Thursday, November 3, 2016 7:20 AM
To: elecraft; KX3 at yahoogroups.com
Subject: [Elecraft] KX3 tuner loss?

I'd appreciate knowledgeable estimates of the loss in dB through the KX3 ATU
when working into a 44ft (NorCal) doublet fed with twinlead, with or without
a BL2 balun inline, on 20/17/15M.

Thanks & 73,
Barry N1EU
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