HF transceiver separate active RX antenna connection
I have only a coaxial going up on the roof, so I can only use a phantom power (power through the same coaxial as the signal).
For the purpose, I have to use an external DC blocking capacitor at the transceiver side, because there is no DC blocking capacitor internal to the machine.
Now my question is what kind of series DC blocking capacitor should I use, based on the fact that it has to allow about 1-30MHz to pass through both on RX and TX and the TX is 100W of RF power?
Shall I use these silver mica (500v rated) ones or do I need bigger ones?
Any suggestions are welcome.
A DC blocking capacitor should have sufficient low impedance (< 1 ohm) in the frequency range of interest, widthstand the RF current of 1.4 A and the applied DC voltage. I'd consider a ceramic chip capacitor, e.g 1 uF/50 V.
The 50v seems too low for me. There are 100W of RF power applied to the 50R antenna, should it be only rated at 50v?
The 1uF seems too high for me. Usually in the low power experiments I use 100nF max at 1MHz and above.
Also I wonder if the chip capacitor would have problems in this kind of application. I would use a higher Q and more reliable capacitor like a silver mica, user on all circuits where power has to pass through or blocked, like the LPF on transmitters. However a practical value for a 500V silver mica will be something like 2.2nF-4.7nF, not even close to 100nF.
I might have misunderstood the intended capacitor function. As described in post #1, you are talking about a series capacitor in the RF feed line, part of a bias T. It should have << 50 ohm RF impedance, respectively doesn't drop RF voltage, only DC. It must be rated for the maximal DC voltage and RF current.
I suggest to sketch a principal schematic and calculate required RF voltage and current ratings.
That's how I read your description
If you accept Xc = 1.6 ohm @ 1 MHz, C must be >= 100 nF. Hence small mica capacitors are obviously not an option.
Yes that's it, but without any T1 in the line.
L1 and L2 should be 1mH or so to block any RF from feeding to the power line I guess.
C2 is of no concern since this will be for receiving only (embedded to the RX active antenna preamplifier).
C1 is my main concern as it will have to pass the whole power of the transmitter to the TX antenna.
I have no idea how to calculate the RF voltage and current ratings you mention, can you help?
I can only guess practically, in the sense that the broadband TX power amplifiers of this kind use silver mica 500V rating capacitors at their outputs of LPFs. Practical values for these mica caps are 2-5nF or so.
U = SQRT(Power*R) = SQRT(100W*50 ohms) = 70V
Most of the HF 100W transceivers use a 100nF/100V at the antenna output.
A 500V mica capacitor would work at 1kW, as a series output capacitor at 50 ohms antenna output.
All right thanks for the numbers, very helpful!
I think you are right, with a simple simulation a 2.2nF has a loss of 0.72db at 160m, which is a considerable waste of power on transmit. When i replace the capacitor with a 10nF, the loss is only 0.04db at the same frequency which is negligible.
But I will listen to your suggestion and try to find a better quality 100nF cap for the Xc.
Luckily in the past I had obtained these big 100nF caps (orange one) shown next to the small 2.2nF mica. I think these are mica caps too cause they have a close tolerance.
Do you think I could use them, as these are better quality I think than the disc types or multi-layer.
How do you arrive at 500 V rating? The series capacitor is loaded by RF current, e.g. 1.4A at 100W/50ohm, 4.5A at 1000W/50ohm. RF voltage across the capacitor is only a few V.
Of course under the prerequisite Xc << 50 ohm.
Suggested 100 nF/100V is O.K., if you don't need lower Xc than 1.6 ohm. Critical parameter isn't however the voltage rating but are ESR and ESL. ESR is also determining the current rating in high frequency range.
I didn't arrive to the 500V rated voltage. The number was mentioned in the first post, as a guess if can be used in a 100W PA.
My opinion that a 500V silver mica capacitor it will work in a 1kW PA at the 50 ohms antenna output is not a guess, is a fact..