Coax/WG transition - probe diameter

I need to make coax to WG and WG to coax transitions at 150 MHz. Can I use probe diameter of several millimeters?

Dear htg,

yes, you can. Probe diameter only affects its impedance; probe length is important and should be adjusted for a minimum loss of your transition.

jiripolivka said:

Dear htg,

yes, you can. Probe diameter only affects its impedance; probe length is important and should be adjusted for a minimum loss of your transition.

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The probe diameter also affects bandwidth...
the larger the diameter the wider the bandwidth

the probe length,as discussed in another topic for 150MHz by hgt myself and others should be an electrical 1/4 wavelength


Dave

davenn said:

the probe length,as discussed in another topic for 150MHz by hgt myself and others should be an electrical 1/4 wavelength
Dave

Click to expand...

Should it be 1/4 free space wavelength or 1/4 waveguide wavelength?

should be a 1/4 waveguide wavelength with is slightly longer than a freespace 1/4 wavelength

see here for the formula **broken link removed**

cheers
Dave

Thanks, Dave. Can you give me an idea of bandwidth that I can expect at 150 MHz if I use 2 mm to 3 mm diameter probe?

htg said:

Thanks, Dave. Can you give me an idea of bandwidth that I can expect at 150 MHz if I use 2 mm to 3 mm diameter probe?

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not off hand, there probably is formulae around for calculating that. Its not something I have had to worry about with the antenna work I have done be that VHF, UHF or microwaves. our 144MHz VHF band is only 4 meg wide anyway, 144 to 148, our 430MHz band 20 MHz wide 430 to 450MHz so on those bands, unless I am building a yagi specifically for a small section of the band I would use 3/8 inch aluminium rod/tube for the elements and that gives all the bandwidth coverage I need.
On the 430 MHz band its noticeable if I drop from 3/8 inch rod to say 3mm brass welding rod. the antenna will drop out of tune much quicker as I tune across the band.
For the power levels you stated in the other topic posting I would suggest that would be determining your prob diameter more than the bandwidth you require.
It would be interesting to see if you woud get arcing problems off the fine tip of a small diameter probe rather than one that was say between 1/4 and 1/2 inch diameter ??? very unknown
If someone cant give definitive numbers then if I was you I would just experiment and see what happens, use lower power levels and slowly increase as you observe return losses and look out for arcing off the tip of the probe.

cheers
Dave

Regarding the power (several kW level) at 150 MHz, ohmic losses may become significant. Should I use litz wire for the probe?

Dear htg:

The "Litz" wire was invented to improve wire loss at frequencies below ~3 MHz. It has no sense to use it at 150 MHz.
For the lowest loss at 150 MHz, material choice is important- use copper or silver; aluminum is good, too.
I can follow your development of a waveguide to coax transmission and other ideas; I cannot see any good purpose of using a huge copper square pipe for 2 m wavelength. Reasonable designers use coaxial lines; for extremely high power, a large cross section. You can check coax cable makers for their data on loss vers.frequency, and power-carrying capacity.

If you insist, you will see that you will have to adjust the best matching of your transition; calculations could give you an approximate result only.

Dear jiripolivka:
1) I cannot disclose why I have to use a waveguide, but I cannot replace it with a coax.
2) I am surprised by your statement about litz wire. At 150 MHz in copper the skin depth is 5.4 micron, so it seems clear that using say, 20 micron wire (AWG52) will help a lot.

htg said:

Regarding the power (several kW level) at 150 MHz, ohmic losses may become significant. Should I use litz wire for the probe?

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The probe is only 0.5 meter long, resistive losses are not likely to be a problem.
I would be suprised if litz wire would stand up to several kW it would probably just fuse and melt.
your probe needs to be very substantial to be able to handle the RF currents involved.

if you were really worried about conductivity of the probe, in relation to skin effect, then silver or gold plate a copper or brass probe.


Dave

jiripolivka said:

Dear htg,

yes, you can. Probe diameter only affects its impedance; probe length is important and should be adjusted for a minimum loss of your transition.

Click to expand...

Do you mean ohmic losses or radiation resistance?

neither

return loss, that is, adjusted for best forward power into a matched load


Dave

jiripolivka said:

Dear htg,

yes, you can. Probe diameter only affects its impedance

Click to expand...

What impedance? Radiation resistance or ohmic losses?