What value of inductor?

[VK5LA]

Hi all,
What value of inductor would stop say 100Mhz and upward of RF,passing thorough it?
Or how would I calculate it?
I am messing with a bias T for powering pre-amps and relays up the coax, and I'm trying to work out a value of
inductance to stop RF coming back to the power supply.
Can anyone help?

Thanks
Andy - VK5LA

[VK5LA]

Oh,
I fogot to mention that I have done a fair bit of research on this looking on the 'net and from
the ARRL handbook, but there seems to be a fair bit of conflicting information.
I have experimented with different amounts of turns on a small VHF (F14) Binocular core, and
it all seems to work regardless, but I'm still getting RF in varying amounts back in to the power supply/Radio.
I have had to add a couple of clip on ferrite suppression cores to the cabling to stop the RF.
Ideally I would like to do away with these and have a smaller, more compact setup.
Am I on the right track here or am I always going to see some RF, no matter what?

Cheers
Andy - 5LA

[VK4QB]

Andy,
What you want to stop RF going into your power supply is a low pass filter between th power supply and the coax, and a high pass filter between the transmitter and the coax. You will need the same at the antenna end of the coax to stop rf in the coax getting into the amps relays and to stop the DC from getting into the antenna . Basically you want to sort out the DC from the AC or in this case RF. If you have a look at how the TV boosters work you can see that not very much is required. At the bottom end you need a rf choke from the power supply or control voltage to the coax cable inner conductor bypassed at the power supply end to RF with a capacitor. This choke has a high impedance to the Rf frequency in question but low impedance or resistance to DC. From the tranceiver you need a high pass filter which is really a capacitor large enough to be a low reactance to the RF at that frequency. This is in series with the transmitter so that it blocks the DC voltage to the transmitter These components are needed at the antenna end too. However on the power supply end you should have a bypass capacitor to earth any rf getting thru the choke, and the same at the top end at the relay end where you should have DC. A choke alone will not work satisfacorily. In actual fact you should be able to use a TV booster system quite ok, but you should be able to find what you want in most ARRL hand books.
I hope this helps
73
Brian 4QB

[VK4VU]

Andy,

I've used this circuit quite successfully for a 2m bias-tee

http://producon.com/sm7ovk/biastee.html

73

Rod Preston VK4KZR

[VK5LA]

Thanks for that Rod,
seems like I have been using inductors of a higher value than what is presented here.
I have also had VSWR problems if I tried any sort of capacitance at the inductor to
ground, admittedly the values I was using was much higher than what is shown.
Back to the drawing board

I haven't seen this page on the net either, thanks again!

Cheers!
Andy - 5LA

[VK2OMD]

Hi all,
What value of inductor would stop say 100Mhz and upward of RF,passing thorough it?
Or how would I calculate it?
I am messing with a bias T for powering pre-amps and relays up the coax, and I'm trying to work out a value of
inductance to stop RF coming back to the power supply.
Can anyone help?

Thanks
Andy - VK5LA

Andy,

Absolutes like "stop" get in the way of understanding what you are trying to do.

You are probably trying to:

• pass DC from the bias port to one RF port;
  block DC to the other RF port;
  reduce the amount of RF getting back into the bias port (power supply in your terms) over a very wide frequency range;
  pass RF from one RF port to the other with little attenuation and very low insertion VSWR and over a very wide frequency range.

[Hmmm, looks like another forum where BBCODE does not work properly?]

It is a simple filtering operation.

A series C between RF ports, and that is large enough to meet the above criteria over the design frequency range will fill the role of the HPF between RF ports. For example, a value of 500pF in a 50 ohm system will cause an insertion VSWR of 1.07 at 100MHz.

Don't overlook that the connection between the RF ports is a transmission line, and will cause its own insertion VSWR if it is not 50 ohms and lengthy.

The LPF for the bias port can be very simple. Assume an L network with series L from the RF port to the bias port, and large shunt C to deck. A shunt 1µH inductor will cause an insertion VSWR of 1.08 at 100MHz.

You might think a larger L would be better, but it brings with it unavoidable distributed capacitance that gives it a self resonance at high frequencies. The smaller the L, the higher the self resonance.

I would design for an inductor of say 5µH in series with another of say 500nH... and possibly another smaller series inductor depending on how high in frequency you want the thing to be effective. All these inductors need to have low self capacitance. If I used a toroid, I would not wind around more than half the core.

The shunt C is not critical, a 1000pf ceramic and 0.1µF monolithic in shunt should be effective. All caps need to be sufficient for your intended voltage (including spikes if you are swithing relays for instance).

You can buy these things, but they are outrageously expensive... you would think they were all laboratory grade.

The answer to your next question might be at Single-Layer Helical Round Wire Coil Inductor Calculator

Does this help?

Owen