VHF (144-148) 1/4 wavelength shorting stub construction...

[GaLuPH]

GentlePeople,

Aside from using the MFJ259B to tweak-in a piece of connectorized coax to become a 1/4 wave shorting stub...does anyone have the mathematical calculations necessary for engineering a 1/4 wavelength shorting stub using LMR-400, so that it will be the most effective in the middle of the 2m ham band?

TUVM! (Thank You Very Much)

[raymond345]

Have you tried http://www.hamfourm.com



.

[va3wxm]

Calculate the length required for 146 MHz. Multiply by the coax velocity factor. Fine tune with the 259. Voila!

[Wile E. Coyote]

There is a lot more information about shorted stub filters at this link:

http://www.radagast.org/~dplatt/hamradi ... ilters.pdf

Good Luck

-WEC

[GaLuPH]

Great Guys - thanks for the tips!

BTW, that link is http://www.hamforum.com -- never had seen that one -- Good call, as well as the K6OIK site ---

Well...the formula that I was able to find was for a 1/4 wavelength stub is:

246 x velocity factor / frequency (MHz) = length in feet

...and from the K6OIK site, the velocity facory of LMR-400 = 0.85

...so 246 x 0.85 / 146.01 = 1.4321' -or- 17.1" WoooHooo!



Getting right on it --- now if I could only remember where I put the "259"...



Thanks Guys!!

[Max-trac]

Put a T in line with a receiver on 146, put the 400 on the 3rd port.
Inject a weak sig.
Leave your 400 long after your calculations.
Take a pin and short along the 400 starting at the end and working in until the signal is weakest. Increasing the sig level as necessary. Cut and solder.
It will be broad, so check it at your desired pass freq.
A narrower one can be built using larger coax or hardline.
Or using a capacitor to tune.
Some other neat stuff is here;
http://www.repeater-builder.com/rbtip/

[Max-trac]

Now that I think about it...
I gave you info on a notch filter.
It should be a 1/2 wave length long (figuring in velocity factor).
Is that what you are try to do??

[Max-trac]

If it is notch filters you are trying to build, look here;
http://www.wa7x.com/ki7dx_rpt.html
And here;
http://www.dallas.net/~jvpoll/dup6m/dup6m.html

There was an earlier thread on building shorting stubs (to DC ground the feedline with out grounding the RF).

[GaLuPH]

Thanks to you too Max-Trac --

However...methinks I erred in the formula - one of my good friends corrected me regarding the calculation for a 1/4 wavelength shorted-stub - should have been:

[468 / Frequency (MHz)] = x-feet (1/2 wave calc)

x-feet / 2 = [1/4 wave length in feet] x 12 = length in inches

length in inches x velocity factor = final length

This is how it worked out anyway with the T-connector, stub, and 50 ohm load (on the other side of the T) connected to the '259'

16.something inches @ 146.010

WooHoo!

[firechief40]

Hello, Would anybody know what the length of the coax for the short stub would be on 154 MHZ?

Im wanting to make one of these for our system since the area we are in is VERY lightning prone. We have installed a polyphaser in the lead-in line plus all items in the rack is grounded straight to EG out at the tower(tower is connected to the same ground. The EG rod is the commercial-type electrical-accepted 8' ground rod and all ground wires are clamped to that.

Thanks