"If I used the "external" ATU option on the transmitter, could I then use an increased wire length without the "error" or damage to the transmitter?"
When the external, or as TH words it.."remote" antenna is selected, it bypasses the internal ATU system completely, passing the RF directly to the F connector. The ATU that mounts outside has the tuning apparatus within it and it is not automatic.
I am not sure what kind of results you will get by putting a long piece of wire to the center conductor of the F connector. I would imagine a bunch of local RFI since it is a raw output set for 75 ohms impedance with no tuning at all.
But I am curious to what you discover.
RFB
The transmitter power limiting is internal to the transmitter so although the "error out" dosen't apply when using the remote ATU, I don't think a mismatch would harm anything but the coverage.
The ATU tuner uses the same loading coils as the internal tuner so although some random length of wire may happen to load up, most likely best operation would be with an 8 foot radiator.
Also, increasing the radiator length affects the radiation pattern causing more peaks and nulls in certain directions.
Well according to my schematic of the TH-II, there is no power limiting circuit..just a sensing circuit that looks at the internal ATU peaking. If there is no peak found, the unit simply displays ERR and the unit never peaks in tuning, but it still transmits. The lack of peak tuning by the internal ATU would give the affect of some limiting circuit kicking in, but there is none I assure you.
RFB
Well, looking at my schematic of the TH-II there is some form of limiting on the remote antenna connector.
Follow the first connection to the left of the remote antenna connector to the collector of a transistor. The transistor is between the antenna connector and ground. It would seem that as the transistor is gradually biased on it would serve as a shunt. The transistor input is tied to the feedback loop for the output sensing which controls the bias to the shunt transistor.
The power to the final output transistors is limited by the output from the modulator op-amp/emitter follower. The regulated quiescent output voltage of the op-amp will limit the total power applied to the RF output stage. The instantaneous power swing due to the modulating signal will be limited by the rail voltage of the op-amp and as designed will not exceed +/- 100% modulation.
All things aside, the output transistors will probably take a dead short on the antenna without damage due to the power limiting of the series modulator.
To test that theory, I grounded the remote antenna output to the chassi which puts a dead short across the transmitter output. I left the short circuit on for 30 minutes. The result was no affect. The transistors did not heat noticably and the output came back to normal when the short was removed.
It seems that circuit is acting more like a crowbar circuit rather than a regulating circuit of the final's. Those transistors in there can dissipate the short on the output connector considering the output connector is not a direct connection to those finals. There is a toroid matching transformer before either the internal ATU and external output connector, providing a set match impedance for those outputs as well as isolation from the output..be it the internal ATU or external ATU.
This part of the circuit using a toroid transformer to couple the finals to the two available output types makes sense so as to keep those finals from changing due to loading.
I dont see a linear action here with this circuit kicking in protecting the outputs. Only real way to test it would be to loosely tap the finals prior to the matching toroid and then shunt the output connector and see what the final's are actually doing, as well as monitor the control voltage to the crowbar and see if it reacts in a linear fashion when changing the shunt on the output connector from a dead short to a gradual rise to the actual load impedance.
RFB
I would accept "crowbar" as a likely application. Appears to be activated by the same voltage that activates the band switch relay. Now why would that be?
I also tried shorting the "wire" antenna connection to the chassi for a while and again there was no apparent heating or damage to the transmitter.
As a side note, the "simple mod" I posted to improve low end audio response is working great. I also added a netting capacitor to the reference oscillator some time ago and the operating frequency stays rock solid dead on frequency.
"Appears to be activated by the same voltage that activates the band switch relay. Now why would that be?"
Perhaps to avoid confusion to the microprocessor as the unit is turning the drive motor moving the tuning slugs in the internal ATU while the unit changes from the upper section to the lower section of the band...a preventive measure I would think to keep the micro from seeing a false peak from a resonance spike or some other odd thing when the band relay flips.
I looked up the complimentary pair of the finals specifications and the unit is well over-engineered in that section. You are probably right that it is an "idiot proof" feature more than anything else.
The internal matching toroid transformer was a very excellent idea of the design. I only wish the audio portion was already set for a wider cut off rather than the stock 750hz cut off point. But it sounds ok even with such a cut off on the low end at 750hz.
I never noticed any frequency discrepancy or drift in my unit's burn in period, which was monitored with a Motorola RD2012 and HP 8950A taking logging pictures every 30 minutes over a 2 day period. It holds pretty good on its own. It may be one of those runs that had a few factory modifications done to the PLL and oscillator from older units having the FM'ing issues.
RFB
None of my TH TX's drift but most all were a little high (80 to 100 Hz) on frequency. An air variable PC mount trimmer at the reference oscillator zero's 'em right on.
Putting the frequency dead on and a slight touch up of the PLL output transformer eliminated all FM'ing warble.
And I really like the audio mod low end response. It was too simple...
The other day when I was getting out past a few tenths of a mile I was on AM 1700. However, I dropped down to AM 1570 so my stereo console (which only tunes up to AM 1600 can receive it. Now, my range is only about 40' from the apartment (with my antenna hanging out the window. I tried grounding the chasis to the center screw on a wall outlet, and tried wrapping the christmas lights around the antenna wire - no difference. The signal is strong inside the apartment though.
Any tips as to how I can improve my signal?
Thanks,
-Travis
You might try making a home made Isotron. Or run a CC setup if there is limited or no access to a decent Earth ground.
The apartment building itself may be the cause of the short range even at 1710. Either the setup is not getting a good RF loop from the lack of a good ground, or something in the building's construction is absorbing your signal drastically.
Apartments are not an easy thing to work with..at least for an AM setup. An FM setup is not that difficult but if something in the building's construction is absorbing that much of your AM signal at 1710, chances are good it will also affect the FM and absorb it too.
Are there any metal cold water pipes nearby? If so, try a continuity check between the neutral of the power breaker box and the cold water pipe. Hopefully it is isolated and the cold water pipe will be a good Earth ground point.
Also to note about the screw holding the power socket face plate. Sometimes those power socket housings are not metal, and the center screw holding the face plate does NOT connect to either ground or neutral because the outlet itself has boht the neutral and Earth ground already.
Back in the day before 3 prong electric outlets, the only way to get the Earth ground through the electrical system was to connect the Earth ground at the breaker box, and have it travel through the metal conduits to the metal outlet housings with the socket. Those old sockets did have the center metal screw holding the face plate making contact with that ground, hence why those plug adapters with the little green tab to bolt down with the face plate screw worked, and you turned a 2 line outlet into a 3 line outlet sporting the ground we all know today in modern electrical systems within homes and apartments.
If your attempting to connect at this point for a ground, connect to the bottom slug of the outlet, or the larger slot of the two vertical slots.
The key here is to find a way to create a good loop for the RF circuit, be it through CC coupling or a wire or rod. It becomes even more difficult if the walls are lined with chicken fence wire and acting as a Faraday cage.
RFB
What RFB just said is true, UNLESS the AC wiring at a socket is done incorrectly. I have found many cases where a do-it-your-selfer criss-crossed the black and white AC wires and put the hot on the wide plug.
I am not going to quote the electrical code for proper wiring, it's too serious. Check a trusted "how to" manual or have an electrician inspect your wiring.
With AC electricity we are talking life-threatening and fire-causing. Don't tamper.