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Monday, November 16, 2009

1928 Transmitter Tuning

Ross Hull's QST article documents a lot of experimentation done to understand the behavior of self-excited oscillators. Whether Hartley, Colpitts, TPTG or TNT Hull concluded that all of them behave about the same and the same tuning guidelines apply to all of them. In summary, any self-excited oscillator should be run at only about half power in order to produce at acceptable signal.

After the transmitter is on frequency and running at full plate voltage:

1) Tighten the antenna coupling as much as possible. In the case of my transmitter this meant moving the antenna coil until it almost touched the tuning coil. At maximum antenna coupling tune antenna loading for maximum output and note this "maximum output".

2) Back off antenna coupling to 75% maximum power (watts) or 85% output current (RF Amps) retuning antenna loading along the way.

3) Once you are at 75% power (or 85% RF current) output, detune antenna loading by adding capacitance to reduce output another 75% power (or 85% RF current). After detuning check your signal. In some cases Hull found that detuning antenna loading worked best if capacitance was reduced rather than added.

Hull plotted frequency vs load capacitance. His curves show a fairly steep curve around resonance and then they flatten out. When a self-excited oscillator is peaked up for maximum output any changes in antenna loading will have a maximum impact on frequency. At this tuneup point an antenna swaying in the wind (changing the load that the transmitter sees) will have a maximum impact on the signal frequency. Detuning the output the oscillator away from peak output moves the operating point to the flatter part of the frequency vs load capacitance curve. Antenna sway will have less impact.

Sunday, November 15, 2009

1928 Transmitter Signal Quality

Ross Hull's August 1928 QST article, "Overhauling the Transmitter for 1929", is a great article for anyone interested in putting a late 20s/early 30s self excited oscillator on the air. This includes the Hartley, Colpitts, TPTG and TNT transmitters used in the AWA Bruce Kelly 1929 CW Party. In the words of QST, this "is, we feel, one of the most important articles ever published for the radio amateur. Let every amateur study it most carefully, and apply its information, for it contains salvation for 1929." It can be found by searching the QST magazine article archives at http://www.arrl.org/.

Ross Hull's major point is that the poor signals legal in 1928 will no longer be acceptable in 1929. He further states that the main problem is the "men pushing the keys". How 1929 transmitters are tuned up makes a big difference in signal quality. The actual transmitter required to meet 1929 standards can be fairly simple. To prove his point he describes the one I built, a "simple -- in fact crude -- rig...capable of performing quite creditably". He even goes on to say that he anticipates some "raspberries" over the crudity of his simple transmitter.

Transmitter features helping signal quality include a heavy tank coil/circuit, mounting the tube so tuned circuit heating is minimized and high tuning capacity/low inductance. Ross Hull emphasizes that the inexpensive hardware features he's listed combined with proper tuning over come lots of the frequency shift introduced by marginally filtered or unregulated B+ supplies, antennas swaying in the wind and components heating up. Simple 1929 transmitters don't need to "splutter, wobble, creep and rattle across great slices of the bands".

In fairness, I still find that moderate winds causing antenna sway does introduce some FMing, but my signal is acceptable. Hull, in fact, states that a DC (T9) note is extremely uncommon and rather unpleasant. He much prefers a more "musical note" that we might rate T8. Given proper tuning that's about where mine is: a sweet, musical, T8 signal. Also keep in mind that this transmitter design does not address other "creature comforts". It is microphonic requiring it to be placed on a table separate from the operating position, hand capacity is a problem and bandspread, such that there is, is barely adequate for getting on frequency. These could be addressed in a more expensive design but their solution is not required in order to archive "any reasonable [signal quality] standard set for 1929".

Friday, November 13, 2009

1928 Hartley Coils


Pictured are my 80 and 40 meter tank coils. Ross Hull made a strong point in his QST article that these coils need to be able to handle a lot of current (5 amps of RF for this design), be mechanically stable and tune with as much capacitance as possible.

These coils were wound out of 1/4" OD copper tubing. A piece of 2 3/8" OD pipe was used for a winding form. Once a coil was wound I installed it and then adjusted the turn spacing so that the bottom edge of each band fell close to maximum tank tuning capacity.

While shopping for copper tubing I found that tubing wall thickness varied brand to brand. For best current handling and mechanical stability I bought the heaviest/thickest I could find.

Friday, November 6, 2009

More 1928 Hartley

The last few evenings have included some final cleanup of the Hartley and then getting it on the air some more. There's been several of us on 80 meters around 3615 during the late evening but the 80 mtr propagation god hasn't smiled on me since my QSO with VE3AWA. Some of the others have heard me but I haven't worked anyone. Here is how I sounded last night at VE7SL's QTH in British Columbia: