DANA's TX BLOG ---------------------------------------------------------------------------- 04 MAY 2015: Transmitter / antenna tests at 5.125 MHz First, as a familiarization exercise, we operated TX4 into a dummy load. One principal object was to establish rough levels for harmonics and/or spurs, using the incident port on one of the 60-dB couplers in the output line from the big PA. These couplers have a 4-inch interaction length, so the voltage response vs frequency should approximate a sinusoidal curve, with zero response at DC, rising to a first maximum at 737.68 MHz. The coupler response is stated to be -60 dB at around 5 MHz, so 100 kW forward power should yield about +20 dBm out of the coupler. During testing, this appears to have been borne out nicely in agreement with power readings from the TX's built-in metering. However, this TX would trip out when we drove hard enough to much exceed about 80 kW indicated output (not a calorimetric measurement). The drive level applied to the installed TX driver cable was about -2.3 dBm for the maximum non-trip output. This is about 589 uwatt, or about 171.6 mVrms, about right for the phase synthesizer's roughly 0 dBm nominal output. One test we performed was to examine the noise spectrum from the transmitter when HV was first turned on, to see if there was any evidence of peaking, which could indicate the presence of sufficient output-to-input feedback to threaten oscillation. Not surprisingly (since we were feeding a dummy load), no such effect was seen. We could see 2nd and 3rd harmonics along with the fundamental, growing to entirely plausible levels at maximum accessible power. In order to look at higher frequency issues without overloading the SA, we added a 75-MHz high-pass filter in front of the SA, and we found we could operate the SA with zero RF attenuation and preamp on without overload. Noise floor was in the area of -100 dBm in (1 MHz or 300 kHZ?) bandwidth, but there were several fixed narrowband spurs ranging from ~100 MHz to 129 MHz, the latter being the strongest. If the observed floor was due to the transmitter's broadband noise and was seen in 300 kHz BW, this would correspond to very roughly 10^-18 watts per Hz. If the beam aperture of the 305m telescope is taken to be 31,400 m^2, then this would amount to about 3000 Jy, a whopping level indeed. However, we don't know at this point whether or not the floor seen was created by TX noise, or was the SA's floor. If the latter, there may still be hope, although a very slim one IMHO. More disturbingly, there was also intermittent broadband noise occurring in flashes, starting at the cuton frequency of the HPF and going to an indeterminant upper limit (it quit doing this before we could find out). When the SA was set to max-hold and the spectrum of this disturbance was allowed to fill in, the envelope looked scalloped with deep nulls spaced at intervals of about 13 MHz. The cause of both the disturbance and the odd spectral envelope remain mysteries. We then moved on to TX1 with TX2, driving the HF-1 crossed dipole pair. The monitor connection was moved to TX2. We first repeated the TX noise test looking for peaks, but found nothing except a single tone at about 109 MHz. This in fact remained even with the HV shut off. When RF drive was applied, TX1 behaved fairly rationally, although its indicated RF output level was unexpectedly high for the drive level applied- about 7 dB higher than for TX4. A drive level of -15.5 dB yielded a metered output of 5 kW, when 1 kW was all that was expected. The internal meter's indications of FWD and REFL power yielded a VSWR of about 1.65:1, pretty much in line with VNA measurements taken recently. However, both the built-in metering and the monitor output for TX2 agreed that the RF output was way low, by about 30 dB according to the monitor connection. This remains unexplained. Having run out of telescope time, we decided to terminate the activities for the day. ----------------------------------------------------------------------- 06 MAY 2015: More transmitter / antenna tests at 5.125 MHz TX2 (HF-1B) still not producing significant output. We began by repeating tests on TX4 into the dummy load, but today we measured its output power calorimetrically. The results were much better, just a shade below 100 kW. So now we know that these transmitters can perform well. We also established that the temporary monitor connections to the directional coupler stack in the transmitters were *not* done correctly. This was straightened out. Running TX1 (at fairly low powers, up to ~5 kW) into dipole HF-1A, we learned two disturbing things: 1. This transmitter, too, seems to put out random bursts of wideband noise, at sufficient magnitude to preclude useful radio astronomy at 327 MHz during the bursts. However, I feel there is a modest chance that in this case the disturbance was originating in lightning discharges. However, this explanation holds very little hope for Monday's phenomenon seen on TX4, since that was seen on the transmitter's monitor output while we were transmitting into a dummy load. 2. At only a few kW output, harmonics of TX1 were strong enough to cause severe interferene to radio astronomy. Of course what matters more for using the HF transmitter is what happens in the way of interference to the 430-MHz radar receivers. The tests at 327 were mainly to decide if there's much risk of damaging the 327 receiver HW. But we could thoroughly eliminate that risk by making the 327's protection RF switches separatly controllable from the 430's ANT/LOAD switch, and instead running the 327's switch in parallel with all the others. We'd probably also want to arrange for all these to be go into the "protect" state whenever conditions warrant, perhaps as simply as whenever HV is on in any HF transmitter. This would preclude operation of HF transmitters even into dummy loads when 305m observations are going on, but I sense that we've already decided to make this policy to avoid the risk of RFI to observations. During idle time I also measured the various filters that Phil uses (30-MHz BPF and 75-MHz HPF) during the HF tests. Files generated were: 30MWIDE.PNG Screen shot over wide span showing the limited re-entrance free frequency range. Filter transmission |S21| is shown in AQUA color. 30MNARR.PNG Screen shot over narrow span showing the pass- band region (has approx 1 dB I.L.). Filter transmission |S21| is shown in AQUA color. 30mhznarrow.s2p 30mhzbpf.s2p These two capture various aspects of filter response as fine-grain tabular S-params. 75mhpfcold.s2p These two show the useful frequency range 75mhpfwarm.s2p of the 75-MHz HPF as well as the curious variation in the response above the useful passband, differing only in a modest case temperature difference effected by holding the filter body in the hand for ~30 sec. 75MCOLD.PNG Screen shot of the filter characteristics showing the useful frequency range and part of the non-useful range (filter body COLD). Filter transmission |S21| is shown in AQUA color. I.L. is approx 0.5 dB for this filter in the "good" part of the passband. I also checked the frequency response of all the couplers on the "test periscope" which we keep assembled on the TX bldg floor for convenience. Files generated were: 60cplr1.s2p These files capture the S-parameters of all 60cplr2.s2p four couplers' "incident" ports (according to 50cplr1.s2p "Dana's usage", not per the labels). This was 50cplr2.s2p done to check accuracy and consistency of the couplers' coefficients and to find out how much the coupling differs at other frequencies of interest. These all showed coupling roughly 1 to 1.5 dB weaker than labeled values, but much of this is probably attributable to the fact that the marker values taken were very near the bottom end of each coupler's "calibrated" band where coupling is expected to be the weakest. 60CPLR2EXP.PNG Screen shot showing coupling coefficient of one of the 60-dB couplers over the HF regime of intest. From examination of the S2P file data, I can say the following: Coupling coefficient of the "-60 dB" couplers rises to about -24.2 dB at 430 MHz. Coupling coefficient of the "-50 dB" couplers rises to about -14.6 dB at 430 MHz. So for both flavors of coupler, the coupling coefficient rises about 35.3 dB going from 5.525 MHz to 430 MHz. SPECIAL NOTE on the directional couplers and their application configuration: These transmitters have a special line section at their output which comprises four separate directional couplers- two each 60-dB and 50-dB. In each of these, both ends of the auxiliary line are brought out to type N connectors. For couplers of this type to work at all well, careful attention must be paid to accurately terminating the auxiliary line. If a given coupler is to be used for measuring power flow in only one direction, it is sufficient to accurately terminate only the "unused" end of the aux line. The end of the aux line that goes into the measurement instrument can be hooked to just about anything impedance-wise without degrading the coupler's directivity. Most commercial couplers are supplied with a good termination permantly attached to one end of the aux line, just to keep the unwitting out of trouble. And this is what is done in the HF transmitters as received, and the whole assembly is labeled accordingly. Except that in this case, the supplied terminations are easily removable. Thus, the whole assembly nets, in effect, two bi-directional couplers, one of which is used for internal monitoring and the other of which is made available to the user for his own nefarious purposes, and nobody has to pay any particular attention to impedance matching of the measurement stuff to 50 ohms. However, the feedpoint current phase monitoring system relies on accurate relative measurements of both forward and reflected waves on the main line. I felt that this need could best be met by using both ends of a single coupler's aux line, with the proviso that I would take responsibility for accurately terminating both ends of the aux line while extracting the signals needed for the phase monitoring system. This approach leaves one coupler still uncommitted for "user" useage, but adds the new requirement that any such user also be careful about the aux line terminations (one of both depending on whether he wants single or bidirectional measurements, respectively). Unfortunately this has led to some confusion in connecting things up inside the transmitters' outer cabinet, but we're working on that. ------------------------------------------------------------------