K1TTT Technical Reference
Subj: Re: Stub QTHs
Date: 96-03-28 13:48:46 EST
In a message dated 96-03-28 08:37:23 EST, you write:
>Question 1): it would seem that placing the stub as close to the linear is
>the best place for it (I guess)....but it sure would be easy to place a
>t-connector on the outside of the house where the hardline ends and take
>rolled up stub line and hang it up on the outside wall of the house on a
>graden hose hanger....it would also be very cosmetic/neat/orderly.
The best place to put a hi-reject stub is exactly a 1/4 wl from the source,
if the source has a low pass filter in the output (like an ampolifier). The
reason is a shorted stub is a low impedance, if you just place it across the
amp output the low shunt Z of the stub barely improves the bypassing. If the
stub is pl;aced 1/4 wl away (at the harmonic F) the transmission line inverts
the impedance to a high impedance. We not only have the advantage of a low Z
stub shunting the line at the stub location, we have the andvantage that the
amplifiers tank (a low shunt Z at the harmonic) looks into a very high Z load
at the harmonic frequency! The improvement in supression can be many dB!
(should shorted stub is low impedance be: open stub is low impedance? - tw)
>Question 2): Okay I 'm gonna put the stub/trap outside. Is there a length of
>coax running to the rig from the stub that would be better or worse? I know
>some have talked about using two stubs and a crtical spacing between the two
>exists (is it 1/4 wave or 1/2 wave ??).
1/4 WL at the harmonic fy, always! Unless the source is high Z at the
harmonic, like a series "C" T network!
>Question 3): If I make the stub out of 1/2" CATV hardline what is my
>multiplier for calculating its length? If I understand correctly, hardline
>a much better choice for this type of a "filter", as opposed to say RG213
>with a woven shield. Hopefully the stub I will need is an "open" at the far
>end and I can home in on the correct length with my tubing cutter and a pair
>of lineman's pliers.
The shield QUALITY has little to do with anything except as it affects line
loss. A lower loss line will present a more extreme impedance at the far end.
Use a low loss line for the stub! Loss is critical to performance.
>Question 4): Have you used Autek RF-1 to make stubs? What technique do you
>use with it, i.e. what position do you set the meter in, etc. Have you found
>it to consistantly say the same thing - and - have you checked it against
>another measuring device. I like ours a lot, it is neat, but I sometimes see
>it "take off" on the lower frequencies, I suspect due to the fact that we
>have an AM broadcast sire nearby which probably is overloading it.
I use a 259 MFJ, but the Autek or anything else will work. BC stations are
always a problem for these instruments. The MFJ 259 instruction manual covers
it all in great detail (it was written by a contract engineer well informed
about RF). Buy the manual, even if you have an Autek!
73 Tom W8JI
Subj: Re: Stub QTHs
Date: 96-03-31 11:55:51 EST
Like anyone would be interested, but I reviewed the data again.
Optimum attenuation occurs when cascaded stubs are place 1/4 wl apart (or odd
multiples thereof) *at the REJECT frequency*.
Optimum SWR bandwidth *at the pass frequency* occurs when pass stubs are 1/4
wl apart at the pass frequency.
Optimum harmonic suppression, if the source favors a low Z load at the
harmonic's frequency, occurs when the first reject stub is 1/4 wl away from
the source at the reject frequency. This is the usual case, and varies with
the PA's internal layout more than anything else.
Optimum harmonic suppression, if the source favors a high Z load at the
harmonic's frequency, occurs when the first stub is right at the output port.
This is a rare case.
I can't find any exceptions to these general statements.