A correspondent asked for a walk through of use of a couple of my online design tools for a 6m 350W single ended valve PA using three QQE06-40 valves. The request was perhaps inspired by a design he had seen, but I sound a caution about a large number of parallel valves (6 sections in this case).
Key design parameters:
HV power supply fully loaded: 1200V;
Power output: 350W;
Vak min: 180V (from datasheet anode curves);
Pi output network, Q at least 12 (for reduction of harmonics on the FM broadcast band), select 15.
The datasheet gives max supply voltage at 450 for plate and screen AM, which implies max ‘instantaneous’ DC supply voltage in AB1 SSB telephony of 900V… so 1200V goes beyond the guaranteed ratings. Of more concern, it is probably close to 1400V lightly loaded, 56% greater than the maximum instantaneous supply implied by the AM specifications.
output network efficiency 90%;
valves load share equally.
An advantage of a high Q design is that it requires higher input C which makes accommodating the self capacitance of the 6 valve sections somewhat easier. A disadvantage is lower efficiency.
There seems to have been a revival in use of the so-called End Fed Half Wave antenna.
The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.
Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.
A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (0.005,13).
Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima. Continue reading Thinking about SOTA, EFHW and EMR safety
The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.
In an attempt to limit the propagation of posits and hence the probability of corruption / delay etc, I have experimented with a path of WIDE1-1 on a recent trip to Canberra (about 400km for the round trip).
Whilst this should prevent packets getting to the Wagga, Newcastle and Tamworth regions which have been the main cause of corrupted posits and mapping defects, it does so at the risk of some loss of posits as some digi infrastructure was never updated to the “New N paradigm” of more than a decade ago and they ignore WIDE1.
Above is a zoomed in view of the Canberra end of the trip, and I am pleased to say that the zig zag double backs that have been evident in recent trips did not occur. The principal reason is that with a path of WIDE1-1, the packets did not pass through VK2RWG-1/VK2KAW. Continue reading APRS duplicate removal – trial #3
Much is written about ATU efficiency, about the need for them or not, and often in subjective terms like “lossy ATU”, and most of it lacking quantitative detail.
The little quantitative detail is almost entirely for purely resistive loads… as if that is typical of real life conditions.
The most common configuration used today is the ‘high pass T match’, but a range of other configurations are seen as being superior… though usually without quantitative evidence.
More Hams use MFJ-949s than any other antenna tuner in the world! Why? Because the worlds leading antenna tuner has earned a worldwide reputation for being able to match just about anything.
… so let’s make some measurements with a reactive load on a MFJ-949E. Capacitive loads tend to be very common for antenna systems at lower HF, so let’s choose a load of 50Ω with a 100pF silver mica cap in series at 3.6MHz. The reactance of the cap is -442Ω, so the load is 50-j442Ω, and the 50Ω part is a RF power meter (RFPM1).
The test setup then is:
a standard signal generator (SSG) on 3.6MHz with 20dB precision attenuator so that we are confident that Zs=50Ω (important to the adjustment of the ATU for maximum power as indication of 50Ω match);
The SSG was adjusted for -10dBm out directly into the RFPM1, then the ATU+cap inserted and ATU adjusted for maximum power indication. Power indicated was 1.4dB lower, so InsertionLoss and TransmissionLoss are both 1.4dB.
Above is a simulation of the T network in RFSim99, component values are adjusted for a match and inductor Q is calibrated to the measured loss of 1.4dB. Continue reading ATU efficiency
Eric posed a quick quiz for the masses to test their knowledge under his heading “Do you really understand impedance matching?”
For your convenience, I will quote his challenge here.
All connections are made with low-loss coaxial cable. The antenna tuner is high quality with negligible losses.
According to standard conversion charts, we find that 4:1 SWR will give us 36% reflected power. Keep that number handy.
Now, we set up the experiment. First, set the slugs on BOTH wattmeters to read REFLECTED power.
Turn on the transmitter, and adjust the antenna tuner for zero reflected power on Bird #1. Switch to forward power, and set transmitter output to exactly 100 watts. Readjust antenna tuner if necessary to achieve zero reflected power, while maintaining 100 watts forward.
Go to Bird #2 and confirm that reflected power is 36 watts.
Question: What is the FORWARD power on Bird Wattmeter #2? How you answer this question determines if you understand the conjugate match theorem or not.
Let us assume that the transmission lines are 50Ω, and that the Bird wattmeters are calibrated for 50Ω.
Above is a plot from that article. I cannot be sure what version of Antscope was used to create the graph, but it was no later than v4.2.57, as one of the ‘improvements’ of v4.2.62 and v4.2.63 was to reduce zooming of the Z scales to a maximum of 600Ω. Continue reading Rigexpert’s Antscope takes a step backwards
(N6PAA nd) describes several small transmitting loops (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.
This review focusses on his 40m STL.
The loop is a circle of perimeter 3.83m which at 7.1MHz is 0.091λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 1.5m above real ground, so we expect a significant ground loss resistance component in Rtotal.
N6PAA gives a measured VSWR curve for the matched antenna, and the VSWR=3 bandwidth as scaled from the graph as 20kHz, from which we can calculate the half power bandwidth and eventually, efficiency. There is some suggestion that some measurements were taken indoors, this analysis assumes that the relevant measurements were taken outdoors as pictured. Continue reading Review of N6PAA’s 40m STL