Resistor noise at RF frequencies?

[MikeMl]

I am building a remote coax switch containing a relay to switch between HF dipoles. Power level is <1kW at 3 to 30MHz.

I want to put a resistor or RF choke from coax-center-conductor to shield (RG-8X) to drain any static charge while the inactive dipole is open-circuited. The active antenna is DC grounded through the relay by a balun between the antenna switch and the transceiver.

What resistor should I use? Carbon or Metal Film? Resistance? Wattage? Does the resistor raise the noise level the receiver sees? What value of resistance is high enough not to heat during transmit while still bleeding off static?

Should I not use a resistor at all; rather use an RFChoke with a low DC resistance?

 

[JimB]

An interesting question.

I would be inclined to use a carbon film or metal film resistor. My first guess was that a 10k ohm resistor would be good starting point.

Using the accepted formula for rms noise voltage v = sqrt( 4.K.T.B.R)
Where K is Boltzmans constant 1.37 x 10^-23
T is the temperature in degrees Kelvin
B is the bandwidth in Hz
R is the resistance in Ohms

I assumed a 3kHz bandwidth, an a temperature of 40degC (Arizona ?)
That gave a noise voltage of 0.7uV.

That 0.7uV is going to be swamped by other noise on most frequencies below 30MHz.

As for power rating, assuming 1kW of RF, that gives 700volts in 50Ohm system.

700volts across a 10kOhm resistor is 5 watts. You could reduce this by making the resistor 100kOhm, then the power dissipated would be only 0.5 Watts.
But now the noise voltage would be increased to about 2.2uV which you would probably notice on the higher frequency bands, certainly on 10metres.

Another thought occurs to me.
You are building a changeover box with relays, why not wire it so that the de-selected antennas are grounded, either through a resistor or a plain old short circuit?

JimB

 

[MikeMl]

The relay that has large spacings for ~1kV is only 1 pole form C. I suppose I could look for a DPDT relay...

 

[RadioRon]

I'm not clear on how resistor noise couples into the rest of the circuit. Jim's comments imply that all of the noise voltage from the resistor appears across the 50 ohm input of the receiver. Let's assume that the antenna transmission line has an ideal 50 ohm antenna at one end and an ideal 50 input port to a receiver at the other end. The resistor equivalent circuit for our purposes would be an ideal voltage source in series with a noiseless ideal resistor. When you place this across the transmission line, the noise voltage suffers from the voltage divider that is all these resistors, and so impresses a voltage across the receiver terminals that is scaled by the ratio of the resistors. In other words, Vrx=25 ohms/10Kohms. This implies that the 10K example would put 1.75 nV across the receiver terminals. Is this not the case or do I have it wrong?

 

[cct314]

Were I curious about this I would:

- Listen to the noise level with a 50 ohm resistor/dummy load across the receiver. Disable the AGC (or turn up the RF gain to effectively do so) and connect an AC voltmeter across the speaker if you want.
- Now, repeat the above steps, connecting a 10k, then a 100k resistor in parallel, in turn, with the 50 ohm resistor.

I don't know about you, but just thinking about putting a 10k/100k in parallel with the 50 ohm resistor makes something inside me say "It's probably irrelevant at HF,anyway!"

 

[JimB]

I'm not clear on how resistor noise couples into the rest of the circuit. Jim's comments imply that all of the noise voltage from the resistor appears across the 50 ohm input of the receiver. Let's assume that the antenna transmission line has an ideal 50 ohm antenna at one end and an ideal 50 input port to a receiver at the other end. The resistor equivalent circuit for our purposes would be an ideal voltage source in series with a noiseless ideal resistor. When you place this across the transmission line, the noise voltage suffers from the voltage divider that is all these resistors, and so impresses a voltage across the receiver terminals that is scaled by the ratio of the resistors. In other words, Vrx=25 ohms/10Kohms. This implies that the 10K example would put 1.75 nV across the receiver terminals. Is this not the case or do I have it wrong?

I am glad that you mentioned that Ron.
I had been thinking that what I had written would be correct if the input impedance of the receiver was high compared with noisy resistor.
As the noisy resistor is in parallel with the 50 Ohm (maybe) input of the receiver, the noise voltage will be divided as you have so well explained.

Of course, as is the way of things, I forgot all about coming back to this thread to voice my second thoughts.

So,
Ron, thank you.
Mike, just fit a 100k resistor. (I hope that you have not spent a fistfull of dollars on more relays).

JimB

 

[MikeMl]

Good point about the 100K noise source being shunted by (now that I think about it) by a 50Ω. Whatever noise the resistor makes is reduced by 20 log(50/100000) = -65db...