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G8MNY  > TECH     15.09.19 08:33z 114 Lines 5588 Bytes #999 (0) @ WW
BID : 21673_GB7CIP
Subj: Coax Traps
Sent: 190915/0824Z @:GB7CIP.#32.GBR.EURO #:21673 [Caterham Surrey GBR]

By G8MNY                                  (Updated Nov 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

To make a coax trap to reject a frequency you basically just T any piece of low
loss coax with electrical length equal to a 1/4 wave for the notch frequency
needed, across the aerial feeder/socket.

 Aerial            "Teed"
  \³/     feeder    notch     Rx
                  Open coax

Leaving the teed coax "open" a 1/4 wave back from that where the T is you get
the opposite impedance "a short circuit", & it this that gives the attention
across the feeder. So the lower loss in the trap coax the deeper the notch.

To determine the physical length the coax velocity factor is needed (how much
shorter than an actual 1/4 wave the coax has to be). e.g. 0.66 for clear
poly & 0.78 for air spaced & foamed (white) poly.

Cutting the length too long & trimming it shorter at the lower frequency
e.g. 144MHz not 145MHz to start with, so you know the depth of the notch then
trim on frequency until the signals attenuates at the wanted frequency.

  Starting point            Done             Overdone start again!
 ³---.   .----         ³----.   .---           ³-----.   .---
 ³    \ /Too           ³     \ /Just           ³      \ /Too
 ³     V Long          ³      V Right          ³       V Short
       145                   145                     145

This is more difficult if the trap is for say a 158MHz pager Tx to a 2m Rx,
than say 145MHz to VHF Rx, unless you have signal generator & a Rx for the
frequency (scanner). See multiple resonance below.

Using a 1/4 wave coax stub on 144MHz also works on 3x @ 432MHz, but unwanted
notch @ 5x 720MHz. This may not be what is wanted for a UHF TV Rx !!
  0dB³Ä-._     _.-ÄÄÄ-._     _.--ÄÄ--._   _.--ÄÄÄ--._   _.--
     ³    \   /         \   /          \ /           \ /
-10dB³     | |           | |            V             '
     ³     Ž Ż            V
-20dB³      V             
            1      2      3      4      5      6      7     1/4 Waves
           144           432           720           1008   MHz
Of course leaving a teed piece of coax across a Tx feeder would upset the SWR!

On a 2 Tx site use can be made of this multiple notch, to stop PA inter mixing
instead of circulators, if the 2 Tx are a fair frequency apart. The open
circuit coax length needs to be approx a 1/4 wave at the difference frequency &
an even number of 1/2 waves at each Tx. Or say a 400W pager Tx on 158MHz &
144MHz nearby ham with Rx QRM, coax of about 0.66x 1/4 @ 14MHz = 3.5m long cut
back to 3.446m for reject & 3.4m pass.
                                                           Tx    QRM
³-._     _.-Ä-._     _.-Ä-._     _.-Ä-._     _.-Ä-._     _.-Ä-._     _.-Ä
³   \   /       \   /       \   /       \   /       \   /       \   /
³    | |         | |         | |         | |         | |         | |
³    Ž Ż         Ž Ż         Ž Ż         Ž Ż         Ž Ż         Ž Ż
³     V           V           Y           Y           Y           Y
      1     2     3     4     5     6     7     8     9     10    11   Waves
      14                                                   144   158    MHz

With heavy low loss coax this approach can give a QRO trap that is open circuit
on the Tx frequency, but 20dB down on a nearby QRM frequency. To set up the
coax is just TEED to the Tx line with a Rx looking at the QRM instead of the
Tx then the line is trimmed to null out the QRM. On Tx of the line is still
near a 1/2 wave line resonance so on Tx the SWR will still be 1:1.

See also my bul "VHF/UHF TVI Filter" & also "HF Rx Suckout Trap".

However using a short circuit 1/4 wave Teed in a Tx/Rx for its frequency, will
attenuate all even harmonics & protect from static. The shorted line is seen as
open circuit a 1/4 wave away at the T point so has no affect on SWR.

  0dB³     .-Ä-.       _.-Ä-._      _.--ÄÄ--._   _.--ÄÄ--._    _.--
     ³   .'     `\   /'       '\   /          \ /          '\ / 
-10dB³  /         | |           | |            V             '
     ³ /           V             '
     DC     1      2      3      4      5      6      7      8   Harmonics

Feedback from Pete G6KUI...
I've used co-ax traps for many many years, both on RX and TX.
A couple of points.
You really need to use solid dialectric co-ax. If you use semi-air-spaced you
will find the electrical quarter wave will vary as you bend the co-ax stub &
hence the notch frequency will shift as you bend it.

When I was very active on 70cm ATV, I had to manufacture a whole pile of the
stubs for my neighbours TVs. The quarter wave stub plus a patch length of coax
were squeezed into one belling-lee(plug) & a belling-lee(socket) on the free
end of patch length. Final trimming of the stub done in the shack after
assembly. This way the fitting to neighbours TVs was a very simple & quick
operation. A very cheap solution to the problems in those days.

On the TX side of things (2M) - I fitted a shorted stub to my 2M linear,
it got rid of a horrible 2nd harmonic that the linear was producing.

Why don't U send an interesting bul?

73 De John G8MNY @ GB7CIP

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