Topband: cutting coax stubs for 80 meter 4-square
k8gg at voyager.net
k8gg at voyager.net
Sun Jun 10 08:59:10 EDT 2018
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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