13-05-2017, 11:44 AM
Hi Dave and others,
Trix:
The offset between pass and notch.
I have a 4 can set on 70cm here 60mm OD in bad shape in that the cabling,
harness RG303 teflon coax has been floating around in a car and some coax braids have
broken. These cans have a side loop with a cover plate and a short piece of coax going to
a "T" (3 coaxes) for the signal "pass by".
I've removed the antenna "T" (N socket, two coaxes going in) and put on two connectors (for now)
Now, a single can and a set of three.
One can has a 3.5cm coax from the T to the can loop. Thus notch to pass spacing is about 3MHz.
This can has a 21cm open stub from the T. Why??
3 cans have a 7cm coax from the T to the can loop. Thus notch to pass spacing is about 1.5MHz
At 440MHz, a 1/4 wave is 12cm in this coax. Teflon, Velocity Factor 0.69
I have seen a 1/4 wave line from the can loop to the T to swap the pass and notch. I suspect
this should be semi-rigid.
The lines to the antenna T should be 1/4 wave or 3/4 wave if too short practically.
Reason: The notch of a can is when it "shorts" the signal to GND. Thus a 1/4 wave line (or 3/4) will show
an open circuit back at the antenna T so the SWR will be not affected for the signal going down the "other" chain.
The two loop cans pose a problem here: Though the SWR of a can at the notch frequency is infinite, the impedance
could be anywhere on the outer circle of the smith chart. Thus you will see two different coax lengths BUT never
have I seen any discussion on this!!
I made up a short coax with two coaxes going into an N plug. I can then check the SWR and get the coax lengths correct.
I suspect the gurus of old had a zillion coax joiners, male to male and female to female and "built" a cable to length.
GR (General Radio) made an extendible line, probably for this exact purpose. With GR connectors of course. I have one.
It's all fun.
Alan VK2ZIW
Trix:
The offset between pass and notch.
I have a 4 can set on 70cm here 60mm OD in bad shape in that the cabling,
harness RG303 teflon coax has been floating around in a car and some coax braids have
broken. These cans have a side loop with a cover plate and a short piece of coax going to
a "T" (3 coaxes) for the signal "pass by".
I've removed the antenna "T" (N socket, two coaxes going in) and put on two connectors (for now)
Now, a single can and a set of three.
One can has a 3.5cm coax from the T to the can loop. Thus notch to pass spacing is about 3MHz.
This can has a 21cm open stub from the T. Why??
3 cans have a 7cm coax from the T to the can loop. Thus notch to pass spacing is about 1.5MHz
At 440MHz, a 1/4 wave is 12cm in this coax. Teflon, Velocity Factor 0.69
I have seen a 1/4 wave line from the can loop to the T to swap the pass and notch. I suspect
this should be semi-rigid.
The lines to the antenna T should be 1/4 wave or 3/4 wave if too short practically.
Reason: The notch of a can is when it "shorts" the signal to GND. Thus a 1/4 wave line (or 3/4) will show
an open circuit back at the antenna T so the SWR will be not affected for the signal going down the "other" chain.
The two loop cans pose a problem here: Though the SWR of a can at the notch frequency is infinite, the impedance
could be anywhere on the outer circle of the smith chart. Thus you will see two different coax lengths BUT never
have I seen any discussion on this!!
I made up a short coax with two coaxes going into an N plug. I can then check the SWR and get the coax lengths correct.
I suspect the gurus of old had a zillion coax joiners, male to male and female to female and "built" a cable to length.
GR (General Radio) made an extendible line, probably for this exact purpose. With GR connectors of course. I have one.
It's all fun.
Alan VK2ZIW