Hi Neil,
In trying to understand the tuning procedure for the SSTRAN, I learned that things aren't always how they seem. For one thing, when you tune for a voltage peak at the SSTRAN's built-in power-dectector circuit, you are seeing really an impedance peak.
This means that the SSTRAN is tuned not to the series-resonant frequency of the plain antenna (where there should be an impedance minimum), but the parallel-resonant frequency of the combined network.
I just thought it was interesting since one would normally assume that the antenna system is resonant at the transmit frequency, but in this case, it is, and yet it isn't. I also noted that without the parallel capacitance, the waveform across the antenna system was strong in 3rd harmonics, despite being tuned to the transmit fundamental.
However when the capacitance was added back in, the wave looked much nicer (I didn't run harmonic distorition, however.)
Now I need to stop all this analyzing and start laying down a ground system!!
Dave
Dave,
Nice report on your model. I also modeled it using Electronics Workbench (a GUI for Pspice), but I did it a bit differently. I ran a Bode plot which shows the gain of the network vs. frequency and only saw one peak near 1600 kHz. You reported the impedance which may behave differently from the gain, but we both agree that the voltage will be maximized at the "tuned" frequency.
The equation I derived shows the effect you saw with the possibility of the numerator going to zero and the denominator going to zero at different combinations of the elements in the network. This "dual resonance" shows up in crystals and duplexers among other radio circuits.
Your split coil idea should work since you will have control over XL and C5/C23 for adjustment.
Keep up the good work and let us know how it works in practice.
Neil
Dave,
A few posts back in this thread I wrote:
>Now consider that the antenna and coil are not really resonant at the adjustment peak. If true, then the antenna will appear with reactance in the model and C5/C23 with this reactance will form either a L network with the coil inductance or a tapped C network with the antenna capacitance which might act as an impedance transformer and C5 is really adjusting the impedance match and not the resonant frequency.<
Apparently you have confirmed this.
Also, when you look for harmonics or distortion you should look at the voltage across the radiation resistance in the model and not the antenna input since the power dissipated in this resistance is what is radiated. Another way to see this is that this signal is the output of the network.
Now, start digging.
Neil
Hi Everyone,
Resurrecting an old thread!
I was just reading the new thread at http://part15.us/node/1377 which references this thread. I had originally overlooked this thread. Maybe I can belatedly shed some light on the SSTRAN antenna principles.
The SSTRAN output transistor wants to see a load of about 750 ohms, so impedance transformation is required to match a short vertical antenna. This is done by means of a capacitance input L-net. The parallel combination of C23 (560pf) and the trimmer (12-100pf) to ground is the C part and the series L part (about 20uH) is obtained from "part of the loading coil". This sounds strange, but two inductors in series are indistinguishable from one inductor equal to the sum of two inductors. If you want to investigate further, there is a free program available on the Internet for L-net calculations. Search for "L-tuner". Plug in the input impedance (750 ohms) and output impedance (say 50 ohms) and look at the results.
It is important to understand that the loading coil and antenna capacitance form a series-resonant circuit. When the inductive reactance of the coil is equal to the capacitive reactance of the antenna, the reactances cancel and the circuit is resonant. Ideally, a series-resonant circuit has zero resistance at the resonant frequency. In reality, coil loss adds a few ohms resistance.
A short vertical antenna looks just like a capacitor to ground. There are equations to calculate the capacitance based on length and diameter of the radiator. One equation is:
C = 17L / ( (ln(24L/D) - 1 ) * ( 1 - ( F*L/246 )^2 ) )
whare Ca = capacitance of the antenna in pf
L = height of antenna in inches
D = diameter of antenna conductor in inches
F = frequency in MHz
For the SSTRAN antenna, L = 106”, D = .625” and using F = 1600kHz, the result is 31.2pf. The inductance of the loading coil required to resonate with 31.2pf is 317.1uH. Note that the antenna height is limited to 106” for the SSTRAN antenna to allow for the height of the loading coil and connecting wires to stay within the 3 meter limit. 317uH is realized with about 75 turns of #16 magnet wire tightly spaced on a 3.5” OD form (3” PVC pipe).
The series resonant circuit has a very high Q, so harmonics are suppressed very nicely. No further filtering is required. (Q can be further maximized by winding a coil on a larger diameter form).
As I said above, the antenna looks like a capacitor to ground. The bad thing is that the “ground” is not typically very good. Actually, the capacitance of the antenna is in series with the ground loss resistance, which depends totally on the quality of your antenna ground. It can vary from about 70 - 100 ohms for a single ground rod in nonconductive soil to almost zero ohms for a broadcast quality ground radial system (360 quarter wavelength radials). The load that the L-net sees is simply the ground resistance and varies from 100 ohms to near zero (if you are a grounding maniac!). The trimmer capacitor, C5, helps to vary the L-net to adjust to a range of ground resistances, but may not be able to compensate for extremely high or low ground resistances.
Now to add another parameter, the “radiation resistance” of the antenna is in series with the ground resistance. The radiations resistance of a “3 meter” antenna is about 0.1 ohm at the high end of the band. The actual radiated power is the power dissipated in the radiation resistance. So you end up with 0.1 ohm in series with say a ground resistance of 30 ohms. The radiated power is miniscule compared to the power dissipated in the ground loss resistance. This is why it is VERY important to have a very good ground system. Range is highly sensitive to ground resistance.
PhilB
Phil,
Thank you for the very nice writeup on this topic. I hope Dave, the original poster, gets to read this.
The subject of impedance transformation is not obvious when one looks at these circuits and your assurance that this is what occurs is very helpful.
Neil