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Re: Topband: cutting coax stubs for 80 meter 4-square

To: topband@contesting.com
Subject: Re: Topband: cutting coax stubs for 80 meter 4-square
From: Jeff Blaine <KeepWalking188@ac0c.com>
Date: Sun, 10 Jun 2018 22:28:11 +0800
List-post: <mailto:topband@contesting.com>
The MFJ259 is a fine box but when you get to 80m with anything near a BC it's going to be problems.  In fact I discovered the hard way that even up on 20m the box can read funky if your antenna is big enough or high enough.  I have a Rig Expert now as well and really love that thing - plus it's apparently immune to the local BC stations.  Not quite as capable as a VNA but has OSL cal, runs forever on battery and does stuff like stub tuning as you are trying here.  Don't think I have used the '259 since i bought the RE last fall.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

On 10-Jun-18 8:59 PM, k8gg@voyager.net wrote:
Hi All,

I was discussing this thread with a microwave engineer on 75 meters this
morning and he reminded me that I had AM RF in my RX antennas at my
previous QTH.  So much that I used Hi-Pass filters on my beverages and
pennants.

Terry,  Do you have an AM radio station near by that could be messing with
your instruments?

Just a thought.  GL,

de George,  K8GG



Hi guys and gals,


Well, to make a long story short my Rig Experts AA-170 has quite working.
Looks normal but a 56 ohm resistor at the end of the 75 ohm coax reads
infinite. On the MFJ-259b it reads 56 ohms. Check SWR and the MFJ reads
7.499.2 Mhz 1.1:1 SWR R=56/57 ohms X=0. Coax is 55' 4 3/8" long when I got
very near 7500 KHz. With the shorted coax the MFJ-259b read 7.484.2 to
7.523 Mhz, 20.2 SWR, R=2 ohms, X=0. Seems like the velocity factor is
considerably higher than what was stated to me by DX Eng.. The velocity
factor is some where around 0.84401... as best I can figure it. So I guess
that makes it 0.84 VF FWIW.


Anyway, I have it figured that 55' 4 3/8" will do the trick for my 80
meter 4-Square at 3.750 Mhz.

But why my RE analyzer went out today I don't have any idea. Back to the
MFJ!


Terry

KI7M


On June 9, 2018 at 6:59 PM terry burge wrote:


     Good point Wes. I will see if I can find a decent value of resistor.
My Rig Experts AA-170 was purchased because I did not trust the
readings my MFJ-259b was giving me while working on my hytower.
Pretty penny for these antenna analyzers. But I will bet if it won't
do the stub frequency checks it won't do the velocity factor and
more. Did work pretty good for checking SWR and feedpoint impedance
on antennas like the hytower. Really liked it until this.


     Terry

     KI7M

         > > On June 9, 2018 at 5:11 PM Wes Stewart wrote:

         Since he said the coax was 75-ohm, a 75-ohm termination would
be better.

         On 6/9/2018 5:07 PM, Richard (Rick) Karlquist wrote:
         > > As a sanity check, terminate the coax with a 50 ohm
resistor and verify
             > > > that you have good coax. Flat line at 1:1 SWR, etc.
             Rick N6RK

             On 6/9/2018 4:57 PM, terry burge wrote:
             > > > I don't know what is wrong here but I keep getting
in this situation where my
                 > > > > typing starts running back over my writing.
I will try again to reply.

                 Joel,

                 I have tried all that the manual says without
success. All I see is the
                 horizontal solid and dashed lines of R and X. No
indication of a resonance
                 point anywhere from 1880 KHz up to 10,500 KHz. Go
figure? At least my
                 mfj-259b is working old as it may be. Rig Experts
is going to get a nasty
                 note I think. Disappointing, huh?


                 Terry

                 KI7M
                 > > > > On June 9, 2018 at 2:54 PM Joel Harrison
wrote:
                     > > > > > Hi Terry,
                     I replaced the 1/4 lines on my 80 meter 4
square this past fall and used
                     my AA-170 to set the length with no problem.

                     Just select "all" for the mode and set the
freq you're aiming for and then
                     trim the line until X equals zero with the
far end open. Should work fine.
                     I always calculate the length x VF then add
a few feet to start.
                     73 Joel W5ZN > > Hi guys and gals,
                     >
                     >
                     > > > > > I have got DX Engineering 75 ohm
foam coax to make the
                         > > > > > > stubs/feeders for
                         my Comtek 80 meter 4-Square and am
finding some difficulty.
                         Neither my
                         MFJ-259B nor my Rig Experts AA-170
seem to indicate a 1/2 WL at
                         7500 KHz
                         or a 1/4 WL at 3750 KHz? Or any
frequency up or down from there.
                         Seems to
                         me the last time I did this a few
years ago I didn't have a problem
                         getting a 1/2 WL dip indicating a
resonance location. But now I
                         just don't
                         get any indication of a resonance.


                         DX Engineering said they had got some
75 ohm foam coax that
                         instead of
                         0.84 VF was more like 0.80 down to
0.76 VF. But checking all the
                         way down
                         to 6000 KHz on 40 meters or 2500 KHz
on 80 meters fails.

                         My test is to use either antenna
analyzer with a PL-259 installed
                         at one
                         end and an open or a short at the
other. I started with 57'7" of
                         coax.
                         Tried both with the coax in a roll and
strung out. What am I doing
                         wrong?


                         Terry

                         KI7M

                         😞
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