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Author Topic: Evolution of tube transmitter antenna coupling?  (Read 1251 times)
KC9KEP
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« on: January 14, 2011, 07:46:41 AM »

Hello,

The vacuum tube transmitter construction projects featured in the early ARRL handbooks frequently utilized a large primary/secondary coil type arrangement to couple the transmitter output to the antenna.

Then, somewhere in the late 50’s, the popular scheme seemed to change to the more “modern” PI filter for the output section, featuring a plate and load capacitor and a single coil.

I would guess that the PI filter may “tune” sharper and do a better job of filtering out any harmonics?  Also, the PI filter may provide better provisions for load matching?   And, the coil taps can be switched/shorted to accomodate different bands.  But, these are just guesses.

Does anyone know why the one scheme was dropped in favor of the other?

73!
--KC9KEP
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K0BG
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« Reply #1 on: January 14, 2011, 08:36:54 AM »

You more or less answered your own question. One good thing going for loop tuning circuits, is their wide range of impedance matching. Weigh that against band switching with a knob. There are other advantages to both, however.
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KG6YV
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« Reply #2 on: January 14, 2011, 08:49:06 AM »

LInk coupling was popular with push-pull finals which were often the standard prior to the mid-50's.  The transmitters used triodes and tetrodes in push-pull which has by its nature a "balanced output" circuit.  Plug in coils were also the norm at this time. 

With the advent of newer output tubes often strung in parallel for greater power, the output was single ended and the pi-net is a good bandpass filter in the single ended implementation with a wide matching range for the plate to antenna matching.  Simpler to build a transmitter with a switched coil too, you don't have different resonant coils each with a different inductance link for each band.

People got smarter, tube technology evolved too.

FYI
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WB2WIK
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« Reply #3 on: January 14, 2011, 09:07:25 AM »

Also, in the 1940s and earlier nobody had coax because it hadn't been invented yet (it actually came into use during WW2 and became popular after that).

Single wire fed antennas, and balanced line fed antennas, were the norm.

When coax became popular because it's so easy to use, stuff in transmitter design had to change to accommodate low impedance unbalanced systems.
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KA5N
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« Reply #4 on: January 14, 2011, 10:43:46 AM »

Having to change coils and adjust links was a pain.  Also there was the new problem of TVI!
Changing coils in a buttoned up transmitter is a real pain.  In order to reduce TVI problems some transmitters had finger stock on panel and cabinet parts which were removable and the Viking II even had copper screen across the opening for the tuning read out.
With pi-networks you just flipped the band switch and tuned up.  Low pass filters actually worked if the transmitters were buttoned up for TVI.  Even then TVI was still a problem with Channel 6 and the 21 MHz IF strips in some TV receivers. 
Often operating conditions are not chosen for being the "Best" but rather for ease of operation and
price.  Remember when the "superior" Beta VCR technique was beaten out by the "inferior"  VHS
system?

Allen
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G3RZP
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« Reply #5 on: January 15, 2011, 02:56:36 AM »

Coax was in use well before WW2. The difference was that you made your own. The 1936 (and possibly earlier ones) ARRL antenna book shows how. You thread isolantite beads on the 14 AWG wire. You took a pair of '5 & 10' store pliers (because they were cheap!), and filed a notch in the cutting blades so you could crimp the wire each side of the bead to hold it in place, and then fed the lot into lengths of copper tube - I think it was 3/8 inch diameter from memory.

Even earlier, you just tapped the antenna up and down the tank circuit un til you got enough 'draw': advisedly, you had a series capacitor to keep the high voltage off the antenna.....Harmonics? What harmonics!

Coax as we know it depended upon the development of polyethylene, which was discovered in Germany in 1898. Rediscovered by Imperial Chemeical Industires in the UK in 1933, but commercial production did not start until 1939. Heavily shrouded in secrecy because of its importance for cables, more research was done in the US and licences were given (sold?) by ICI  for their production process to Du Pont and the Bakelite Corporation. Yet despite the secrecy, in about 1938 or early '39, tehre was an article in the RSGB magazine by an amateur who was a professional chemist working in the area about the new plastics - polyethylene and polystyrene - and their RF properties and uses....

The other great thing about the pi network was that it reduced harmoinics and by the 1950's, TVI was the BIG bugbear.
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