Need advice: "T" vs. "L" antenna tuner?

Need advice: "T" vs. "L" antenna tuner?

Post by Dean Hemphill, K5 » Fri, 01 Sep 2000 22:13:50

Greetings from insanely-hot and bone-dry North Texas...

In this world of eBay posts, fights and arguments, and the
endless stream of non-radio "spam", how's about we get some
actual useful radio-related discussion going here?

Here's my question:

What are the advantages and disadvantages of a "T" network
versus an "L" network antenna tuner?  I need to build or buy
a tuner capable of handling up to 1 KW, and I'm open to any
and all suggestions.

I have a 28 uH E.F.Johnson roller coil, a Groth turns counter,
a pair of huge 250 pF air variables, a pair of National Velvet
Vernier drives with giant ceramic insulator rings, a Collins
directional coupler/Wattmeter from a military URC-32A 1 KW SSB
station, a Heath SB-line cabinet for my enclosure (which will
match my SB-200 amplifier), and all the time in the world to
actually make those parts into a tuner.

So, let's hear some serious advice, along with reasons why or
why not.  For everyone's benefit, post your responses to the
group, rather than emailing me privately.

73,
Dean, K5DH
Visit my amateur radio web site at http://www.qsl.net/k5dh

 
 
 

Need advice: "T" vs. "L" antenna tuner?

Post by Wes Stewa » Sat, 02 Sep 2000 01:33:38



>Greetings from insanely-hot and bone-dry North Texas...

>In this world of eBay posts, fights and arguments, and the
>endless stream of non-radio "spam", how's about we get some
>actual useful radio-related discussion going here?

This will be refreshing.

Quote:

>Here's my question:

>What are the advantages and disadvantages of a "T" network
>versus an "L" network antenna tuner?  I need to build or buy
>a tuner capable of handling up to 1 KW, and I'm open to any
>and all suggestions.

After reading your resume, I think you might be trolling, but nevertheless I'll
press on....

In the general sense, the L network can match any two values of  loads to each
other, however, to handle a wide range, the network must be 'reversable', ie,
you must be able to have the shunt element switchable to either the input or
output side of the network.  Furthermore, the network Q is fixed and is
determined solely by the values of the two impedances being matched.

If the impedance ratio is tractable and results in a reasonable network Q then
the L will be the more efficient of the two.

The T network can be visualized as two L networks back-to-back, with a 'virtual'
resistance existing at the junction of the two Ls.  In the usual implementation;
two series capacitors and a shunt inductor, the virtual resistance must be
greater than either of the two impedances being matched.

The advantage of the T is that the 'virtual' resistance and hence the network Q
is under our control.  This versatility can be a two-edged sword.  With three
adjustable elements as you propose, there are, technically speaking, an infinite
number of combinations that will realize a match.  One will maximize the loss
and another will minimize the loss; all of the others will be in between.  Your
problem is to determine which is optimum.

With only a SWR indicator on the input side of the network, you cannot tell
which solution yields the lowest insertion loss.  Generally speaking, setting
the output series capacitor to the highest possible value is the best solution,
however, the use of a roller inductor complicates the mix.  This is because the
unloaded Q of a roller inductor (at least the ones I've measured) varies greatly
with inductance setting.  So an "optimum" capacitor setting might result in
lower inductance setting that reduces the inductor Q such that the total loss is
greater than "optimum."

The best solution (IMHO) is to include an output power indicator and adjust the
network for simultaneous input match and maximum output power.

Quote:

>I have a 28 uH E.F.Johnson roller coil, a Groth turns counter,
>a pair of huge 250 pF air variables, a pair of National Velvet
>Vernier drives with giant ceramic insulator rings, a Collins
>directional coupler/Wattmeter from a military URC-32A 1 KW SSB
>station, a Heath SB-line cabinet for my enclosure (which will
>match my SB-200 amplifier), and all the time in the world to
>actually make those parts into a tuner.

The 250 pF won't be enough on the lower bands.

Quote:

>So, let's hear some serious advice, along with reasons why or
>why not.  For everyone's benefit, post your responses to the
>group, rather than emailing me privately.

For some further thoughts on this subject, you might look at my crude homepage
at www.azstarnet.com/~n7ws and select the ladder line link.

Quote:

>73,
>Dean, K5DH
>Visit my amateur radio web site at http://www.qsl.net/k5dh

73,

Wes  N7WS

ps.  I retired from Hughes Aircraft/GM-Hughes/Raytheon last year.  I too was a
RF-Microwave guy here in Tucson.  Don't miss it a bit :)

pps.  I tried QRP operation from my travel trailer during FD from Custer, SD.
My conclusion, Life's too short for QRP. :)

 
 
 

Need advice: "T" vs. "L" antenna tuner?

Post by Philip de Cadane » Sat, 02 Sep 2000 03:04:35

Dean,

Quote:>What are the advantages and disadvantages of a "T" network
>versus an "L" network antenna tuner?  I need to build or buy
>a tuner capable of handling up to 1 KW, and I'm open to any
>and all suggestions.

>I have a 28 uH E.F.Johnson roller coil, a Groth turns counter,
>a pair of huge 250 pF air variables, a pair of National Velvet
>Vernier drives with giant ceramic insulator rings, a Collins
>directional coupler/Wattmeter from a military URC-32A 1 KW SSB
>station, a Heath SB-line cabinet for my enclosure (which will
>match my SB-200 amplifier), and all the time in the world to
>actually make those parts into a tuner.

>So, let's hear some serious advice, along with reasons why or
>why not.  For everyone's benefit, post your responses to the
>group, rather than emailing me privately.

Wes has given you a more thorough answer than I could.

I was taught that a 'T' can match a wider impedance range than an 'L'.
Most all commercial tuners are of the latter. The exception is the range
from Palstar. All their's are 'T' matches and very nice they are to.

Good luck.

(((73)))

Phil  G4ZOW