Date: Fri, 4 May 2012 09:22:54 -0400
From: "Carl" <km1h@jeremy.mv.com>
Subject: Re: [Amps] Antenna tuner loss
Tuners with 250pf max on 160 will always be inefficient, a High C tuner with
500pf will be very efficient and require proportionally less L. The higher Q
results in less bandwidth so there is no free lunch.
Its so much easier to modify an amp to be able to do the matching and toss
the tuner in the dumpster.
Those with autotune amps are left swinging in the wind and need autotune
tuners (-;
Carl
KM1H
## Im afraid you have the the T network info backwards Carl. On
a T network Tuner, with high C and low L, you end up with LOW
Q ! The T network is 100% opposite a PI net. With any PI net,
High C1 + C2 and low L = high Q. On a PI net, low C1 + C2
and high L = low Q.
## A T net with low C1 and C2 and high L = high Q. That’s why
the damn coils in the tuners all burn up in the 1st place. The T net
in 99% of cases is a hi pass filter.... but who really cares. Any harmonic
redux should have been done at the output of the amp anyway.
## Look at Alphas latest T tuner creation. It uses a pair of 7.5 kv
vac caps + huge edge wound roller coil. They rate it at 4 kw... but
that’s only for high Z loads. For real low Z loads like say 8-12 ohms,
its only good for 600W CCS ! With low Z loads, the current is sky high
through the coil, which limits the PO the tuner can handle.
## For real low Z loads, the best answer with a T tuner is to just pad
one or both caps, and use less L. Then you have a low Q network=
less current through the coils.
## I agree though, just fix the ant in the 1st place. At a friends place, we
used a motor driven ceramic vac cap on his 160m vertical. The vert
is 100 ft tall, with T loading at the top. Its then aprx .375 wave long...
and R = 50 ohms. The XL is simply tuned out by an equal amount of
XC provided by the vac cap. SWR is dead flat from 1800-2000
We stuffed +73 dbm into it, and it doesn’t break a sweat.
## On my 40M yagi DE, I installed one tiny coil, made from .375 inch
OD CU tubing into each leg, right at the feedpoint in a box.
Each coil is shorted out with its own ceramic vac relay. Coils are in the
circuit on CW... and shorted out on SSB. Real low swr from
7000-7300. And no, it cant be blown up with YC-156 power levels.
## On my 80m rotary dipole, I use a pair of motor driven, compressible
tubing coils, made from .25 inch OD plastic coated CU tubing. Comes
with 10 x pre-sets, plus full manual tuning + digital readout. Flat swr
from 3200-4100 khz. You cant blow this one up either.
## 20-17-15-12-10m yagis are a non issue for swr these days, so
no mods done to them, except for bigger OD AL tubing used for the
hairpins. On the OWA designs, no hairpin is used.
later... Jim VE7RF
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