40M Vertical Strange Set-Up

[flintstone mop]

Hello All
I have a strange situation with a vertical installation. It's a re-hash of mast parts about 33 feet long.
I was able to get the vert. to be very happy to accept 7.0something to 7.3something from my MFJ analyzer. A very precise dip of SWR in this range of freq. and an unbelievable 0 on Xs ........The explanation from MFJ and Googling is vague and mysterious, about Xs and I hope someone here can clear up the mystery. That Xs is telling me something very important.
The MFJ is connected directly to the MAST and the ground system.

OK another day goes by and the entire dream of almost perfect goes poof.
NOW the antenna displays a broad dip between the two freq points mentioned above. The dip is no longer sharp. It is broad and does not go lower than 3:1 and Xs 30..........The Xs is telling me something. There's a reactance of sorts?
There was rain the day of the change. I pulled the mast off the insulator and it was damp inside.
I am removing tape I wrapped on the top of the mast to allow it to breathe and ventilate. The insulator is hollow to allow any water to drain and let air circulate within the mast.
Would this be a water problem??
I also have a wire radial system connected along with a vinyl covered/welded fence system. (6 ea. 3 feet wide X 50 feet long fence encircling the antenna mast.)
I could separate or use one radial system to test if there is now a capacitance?? causing coupling?? and making for another RF current flow.

what are you guys thinking??? Antenna gurus who use the MFJ analyzer.......reveal some secrets.

Fred

[flintstone mop]

A finding, of sorts.

My buddy the MFJ analyzer has been telling me that I might be near Nirvanna. The impedance of a real vertical is around 36 ohms and that's what I am reading on the MFJ guy. Now it is a 50 ohm device so the Xs is going to reflect the difference looking into a 36 ohm antenna. So, maybe a little tuning needed to make the 50 ohm transmitter happy??
All of the mast joints have now been cleaned and tightened and the anti stuff in the joints. The mast is about 31 feet tall and it doesn't matter which radial system is connected. Both or one or the other.
What I saw earlier might have been a dream.

Any ideas?
Fred

[N8UH]

For what it's worth, my MFJ has been a bit unpredictable at times. I'm not sure what the issue is, but when I was working on my 75M dipole which is way lower and closer to my station than ideal, I tried both my FT-857D and the MFJ. The MFJ was giving me resonance around 3300Khz. The FT-857D happily delivered full power at 3870 with minimal SWR. Usually the FT, like many SS rigs, is a bit cranky when it comes to SWR, so I can only surmise that the MFJ was at fault. I could be wrong though!

I wonder if proximity or imbalances cause the MFJ to deliver false readings? 

[flintstone mop]

You are correct Tim,
MFJ warns about that. It is not a device you hang your hat on. It gives a relative reading with some accuracy and if there are broadcast stations nearby, then all bets are off.

BTW.......there was an old trick to work around this recent finding in my Re-hash of this 32 foot mast. I can wind a small coil of copper tubing and connect between the mast and ground and tap for the lowest 50 ohm SWR and reactance. It is a matching transformer of sorts. So, I am off to Lowe's and then the inverted L is next.
Fred

[KM1H]

Resonance and 50 Ohms are usually mutually exclusive in an antenna.

The rice box is looking for 50 Ohms and it doesnt care what combination of resistance and reactance are present. If you want that 36 Ohms transformed to 50 Ohms then either a matching length of different impedance coax (use a Smith Chart or one of the on line programs) or an ATU in the shack are needed.

OTOH if you use an amp it doesnt care about any small mismatch and is its own ATU.

Carl

[flintstone mop]

Resonance and 50 Ohms are usually mutually exclusive in an antenna.

The rice box is looking for 50 Ohms and it doesnt care what combination of resistance and reactance are present. If you want that 36 Ohms transformed to 50 Ohms then either a matching length of different impedance coax (use a Smith Chart or one of the on line programs) or an ATU in the shack are needed.

OTOH if you use an amp it doesnt care about any small mismatch and is its own ATU.

Carl

I'll give it a shot, Carl. The amp seems to be happy to tune into the rotatable dipole without any nasty problems. And that antenna has higher reactances!

[ve6pg]

..fred..think of my design, as a fan dipole...just that it is standing on it's end..if my main radiator/support was tall enough, it would be a 1/4 on 75...but considering it is a 1/4 wave for 40, the inverted "Ls" provide coverage for 160, and 75...i've never had swr issues, that is, that a little pruning did not take of..

..tim..

..sk..

[ve6pg]

.fred..i dont know how to reply, to a reply, after i have replied !...not too computer savy....
..my vert., is about 125ft away, fed w/rg213...if i was to disconnect the 75, or 160 "L" , i would see a change in swr on 40...but, as they are all tied together at the feed point, i have excellent swr, on 1880, 3725, 7290, all of 15, 6, etc...i do use a tuner here in the shack, if i roam from those freqs...i just change bands, (without out the tuner) and swr is great...but as you did mention, every install is different...

..tim..

(bithin' hot/humid here)

..sk..

[Jim, W5JO]

Ah ha, erratic readings on an MFJ 259.  Could be the diode bridge in the front end of the analyzer is bad.  One or more of them shorted or opened.  Check by putting a 50 ohm resistor in series with a known cap that you calculate the X then look for that X and the 50 ohms.  If you don't see the calculated value you have diode problems.

Static can ruin those diodes.  Before you hook any antenna exposed outside to it, short the center conductor to the case of the MFJ.  If the wind is blowing any substance like sand or snow or something, can create the necessary static to get your diodes.

I forget what they are but I bet you may have bad ones.  Seems as if they may be hot carrier but don't bank on it.  I know MFJ, in their infinite wisdom, uses the ones that will go bad in static.

[ve6pg]

..jim..cant comment on the MFJ, dont know much about it... BUT...the desk is a jumping because i'm listening to the DC-5....man, i gotta tell ya...dave clark 5 will get ya going.....full bore....thank god their are no neighbours here..

..tim..

...god bless the dc-5...

..sk..

[Jim, W5JO]

..jim..cant comment on the MFJ, dont know much about it... BUT...the desk is a jumping because i'm listening to the DC-5....man, i gotta tell ya...dave clark 5 will get ya going.....full bore....thank god their are no neighbours here..

..tim..

...god bless the dc-5...

..sk..

Played their records as a DJ back in the sixties when I was but a kid.  They do get one's blood flowing.  Turn it up will you?

[ve6pg]

..still sounds good today...before they hit it big, they played at a high school dance, where my brother attended...west toronto, in 1962...

..tim..

..sk..