Tanks and Tuners

[K5WLF]

Been studying a bunch of screwmatic diaphragms recently, trying to get smart(er) in preparation for putting the DX-100 on the air this fall and hopefully adding an amp shortly. I'd like to build the amp, but that may not be feasible at this time.

Anyway, I noticed something that I'd not noticed before. All the transmitter and amp skemats I looked at show a pi tank circuit with the plate and load caps in shunt and the tank coil in series. All the tuner skemats (commercially produced tuners) had a "T" circuit with the caps in series and the inductor in shunt.

I'm sure there's a really good reason for that, but it didn't readily present itself to me. Could somebody explain the technical reason for that, please? I'd expected, for no good reason really, to find another pi circuit in the tuners as well.

Thanks in advance,
ldb
K5WLF

[K3ZS]

Read the other posts in "Newbie Vintage Guy Needs Help".    The tube output impedance of the old transmitters are always higher than seen at the antenna or antenna feedline so a Pi net serves to lower the impedance to match the feedline.    The 50 ohm output of modern rigs may have to be transformed to a higher or lower impedance.   The T tuner can do this, it can be looked at as a reversible L tuner, depending upon the capacitor settings.

[WA1GFZ]

a pi network is a low pass impedance step down network.
A t tuner as most build them are high pass step up networks
A high pass filter is useless in attenuating harmonic energy, but will transform impedance.
They are cheaper to build than a T low pass network which are quite useful.
A low pass requires two inductors and 1 cap.

[W2VW]

T tuner = winner cheapest parts : most impedance range.

[K5UJ]

I bought a T network, a Heathkit SA2040 last spring because it was a good price.  I got all the crud off it, (nicotine) and made it look nice and pretty.  Then I read up on T networks like that one and found out they are high pass.  The caps and coil inside were not usable for anything else here, so away it went at the next fest.   

[K5WLF]

OK, I read the 'Newbie Vintage' posts and the responses here, and I think it's starting to make sense. Let's see if I got this right.

The output impedance of the vacuum tube final in the transmitter is several thousand ohms, while the impedance of the antenna system is, ideally, 50 ohms non-reactive. The DX-100, according to the manual, will match an antenna system with an impedance of 50-600 ohms, "providing there is no capacitive or inductive reactance to tune out" (their words). However, we learned in the 'Newbie' thread that the load cap will, in fact, cancel out a reasonable, and probably small, amount of inductive reactance.

If the antenna system either 1> exceeds the impedance range of the pi output circuit, or 2> has excessive reactance of either gender, then we need the "T" tuner which will step up the impedance of the transmitter output and/or cancel out the excessive reactance. I also understand the high pass, low pass differences in the networks.

Next question I guess is, if the antenna system is of extremely low impedance, i.e. below the 50 ohm lower end stated for the DX-100, does that necessitate the use of a pi network tuner (a stepdown circuit) or is there some tolerance in the transmitter pi network that Heath isn't admitting to?

Thanks for the great responses,
ldb

[K3ZS]

In most cases it will be not much lower that 50 and most of time 50 or higher.   In extreme cases like using a mobile antenna or a short antenna on
160M it could be much lower.

[KD7EDW]

I think about at least 50% percent of my technical reading material has to originate from a "free" pile at some Hamfest.

Anyways, I was reading a section of the ARRL Advanced Class License Manual (1986-87 ED) and it talks abut the pi-L tuner network. Anyone built such a tuner?   I assume since it has a pi section front end (like the tanks on finals)and a L section back (like a long wire tuner) Can this design work for both? 

I am interested because I have a need for something that can efficiently match up a number of different transmitters (such as the ARC-5 transmitters) to a long wire antenna.   

[K3ZS]

My old Globe Chief has a Pi-net.    For low impedances on 7 MHz and lower, you have to use a second antenna tap that has a loading coil in series with the output of the Pi-net.    They then call this a Pi-L network.    I think they had that because the total inductance in the regular PI was not high enough.   An external L tuner, coil in series and a variable cap to ground and coax shield, which can be selected between the input side and the output side is all I ever needed to tune a long wire down to a 50 ohm resistive load.   Usually the cap is on the output side unless the "long" wire is shorter than a 1/4 wavelength on the band in use.

[W4AMV]

http://mikea.ath.cx/eimac/eimac04.pdf

See the above for an excellent article on the pi-L network, their origin and its design by W6KEV. The main theme back then, rid TVI.  This article from Eimac service newletter.

[KD7EDW]

Thx for the really nice link - It filled in some gaps for me.