Total posts : 45366
“The circuit feeding the external antenna jack does not have significant low-pass filtering and it is not tuned, so harmonics are to be expected. This is OK because once the coax is connected to the ATU and the ATU is properly tuned to resonance, the waveform should change to a sine wave and there won’t be significant radiated harmonics.”
Not quite. Try hooking one up to a spectrum analyzer and see for yourself that the tuning of the ATU, internal or external, does not “significantly” tune out harmonics. If you look inside one, or even look at the schematic, you will see the unit has a broad filter design in front of the toroid. It’s not very effective, but brings the harmonics to the required minimum of -20db down from the fundamental.
A spectrum analyzer will confirm this. I’ve done it myself and the Motorola R 2012 D and HP 8590 are highly precision spectrum analyzers and comm’s analyzers.
The TH output network is broad band, the internal auto ATU for the wire and the external ATU for the whip does the tuning.
“I have no explanation for why different people are seeing different power with their seemingly identical measurement method. There are too many possibilities to even speculate.”
Let me jog the memories. These TH units are not built with precision parts, all with 10 percent +/- tolerance and mass produced over in China. It was pretty much concluded by YOU GUYS that the reason for the variance in power and audio performance was due to the wide variance in component tolerances. I even agree with those conclusions. I’ve got two TH V5 units sitting here and both perform differently to each other. One has great sounding audio and range, the other..less than optimum for what it is..even it’s own internal audio source sounds like garbage as well as displays crappy waveforms on it’s own.
“As Neil said previously in this thread, multiplying peak voltage by .707 to get RMS voltage only works for a pure sine wave. At the upper limit, for a pure square wave, the RMS voltage is equal to the peak voltage. The RMS voltage for a random distorted waveform can’t be determined unless you have some sort of RF voltage measuring instrument that displays true RMS voltage.”
The unit puts out a sine wave. With a known good scope start probing from the oscillator and through the two buffer stages, then the final. The carrier frequency is not digitally generated. It is a simple PLL controlled oscillator.
Unless a known sine wave test signal is ran through that scope and verified it’s displaying correctly, anything you pipe into it won’t be displayed correctly thus any reading or measurement will be useless. That is an old o-scope..a kit to be exact, and have you had the caps and V and H deflection circuits tested and adjusted properly? It will have calibration adjustments inside. That Heathkit is 3 decades + old, and if it still has the original caps and the internal setup potentiometers not checked for drift in value, that scope will be way off from what it should be displaying. Has the CRT been checked on a CRT analyzer? Being a 30+ year old scope…I bet that CRT has become “soft” and needs a bumping with a CRT checker/restorer.
Remember the old TV CRT’s and when they aged they would display soft, weak and fuzzy? Hmm….go figure!
“Just recalibrating the scope against a known 15 vdc source and still measure 14-15 vpp. The waveform has the same peak-peak value whether I use the AC or DC setting on the scope. I doubt the unit been “souped up” because it still had a realtor’s message on it when I got it from EBAY last week.”
Well all that is doing is validating the V input with the setting of the V input knob. It doesn’t validate if the sweep rate purity is good..ie the fuzzy display at increased sweep rate. Ebay eh…well unless the seller validated and provided evidence that Heathkit was shotgunned with new caps and pots inside, I wouldn’t trust that scope to display anything correctly..not even a known 15VDC source..if that 15VDC source is truly producing a pure flat DC output too! I should also point out that checking for DC isn’t checking for AC, and the AC sampling components in that scope very well could affect the signal applied to be measured if those AC input components are drifted out of tolerance from 30+ years of age!!
Phil is correct. You do need a precision RF measuring instrument to display the true RMS RF voltage, which is a high frequency AC signal..and always a sine wave at the output, or fed into a filtering circuit if the carrier is digitally generated so you have “wave shaping” taking place. Square waves shooting out a transmitter produces TONS of harmonics.
However I think most of the problem is that o-scope. It’s not displaying correctly, even the displayed increase in sweep making the carrier appear as if it’s “fuzzy”. The TH unit doesn’t do that no matter what frequency it operates on. It produces a pure sine wave across the entire band. And because that output is broadband since the thing needs to operate over the entire MW band, it also has to output a pure sine wave across that entire band. The tuning by the internal auto ATU or external one simply brings the radiator to resonance..nothing more.
It’s possible that TH unit has a problem too..probably in the modulator. Combined with an unverified proper operation of the o-scope, you may very well be chasing phantom issues caused by two incorrectly operating devices..the transmitter AND the o-scope.
Process of elimination…verify that scope is operating properly across it’s entire sweep range..ie V and H deflection, as well as “sweep rate purity”. Grab a second scope known to be working correctly and use that to validate the Heathkit’s proper operation, then test the transmitter.
If you get the same results from two different scopes, then the problem is obviously the transmitter..unless black chance strikes twice..which sometimes it does and you got two improperly functioning scopes.
A simple work around to measuring that RF output would be to build a detector using nothing more than a fast switching diode and resistor pad (3 parts), and use a DVM to measure the DC voltage produced by the detector. Google RF sensing detector and find tons of simple circuits to accomplish this.
Good luck and check that o-scope against another and make sure your not running in circles because of an improperly operating piece of test gear!