Topband: cutting coax stubs for 80 meter 4-square
Jeff Blaine
KeepWalking188 at ac0c.com
Sun Jun 10 10:28:11 EDT 2018
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 at 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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