MFJ 915 INSTALLED AT THE ANTENNA'S FEED POINT

[222DBFL]

Can the antenna be tuned at feedpoint without use of 9ft of coax? Answer is yes. So mast becomes counter poise. This will hold true for a grounded mast or tower. Is this not true or am I missing something. If this is the case then an rf isolator can be added as close to feedpoint as possible. I thought I said it in a post a while back that the A99 was a 1/2 wave end fed dipole with a matching network to get the ohms from 1000ohms down to 50ohms. There is a guy that broke one apart and truly "exposed" the antenna for what it is, an end fed 1/2 wave vertical dipole.
Here is the link for the info on the inside of one. Copy and paste the link. It shows the entire antenna and how it's made. So fellas, what is the bottom line? Should or shouldnt the rf isolator be installed at the feedpoint. I gotta go with at the feedpoint after researching the antenna some more. I just used ferrite chokes at my feedpoint and they seem to work well there. Antenna was tuned at feedpoint with a short (18") coax jumper and an MFJ269. All readings were within reason with r=46, X=4 right at the edge of my metal roof. Once it got up to intended height I checked it at end of coax and readings were a bit better with r=48, x=2 vswr, 1.2 at 27.385. This was as good as I got it without doing anything but getting the antenna from a friend, added HD black heat shrink from top to bottom leaving only the connection parts exposed. No fiber glass exposed at all. One tough antenna now I will say that. But it doesn't work as well as my sirio tornado at the same height for sure. There are guys locally that when I went from 18 to 27ft came up 1-2 s-units and we're now easily reachable. The A99 is up at the same height, no ground plane kit and it works okay but I like my aluminum antennas better.

 

[vkrules]

STOP and read this
.http://www.w8ji.com/end-fed_1_2_wave_matching_system_end feed.htm
In fact read it twice.
W8ji explains it nicely .But read the whole thing don't just take one line to support your argument.
A lot of you guys are on the same page, just not on the same paragraph

 

[bob85]

As far as cmc is concerned there is no difference between a conductive mast and coax braid,
You don't want either radiating,

5/8waves are more prone to cmc issues than 1/2waves due to the lower end impedance / higher current flowing into the antenna but 1/2waves can still have cmc issues enough to cause rfi, metal mic grills biting your lips etc,

The common mode impedance of your mast or coax braid is determined by their electrical length and equally as important them been connected to ground or not connected to ground,

1/4wave or odd 1/4wave multiples isolated from ground give low cm impedance & high cmc, = bad length,

1/2wave or 1/2wave multiples isolated from ground give high cm impedance & low cmc, = w8ji's lucky length,



If the mast or coax are connected to ground the impedance is inverted, then we have a situation where,

odd 1/4waves invert the low ground impedance to give high cm impedance & low cmc, = another lucky length,

1/2waves reflect the low ground impedance to give low cm impedance & high cmc, = bad length

When you operate the radio from a psu you always have a path to ground through the mains earth or ground wire to ground rod & that forms part of your coax braid electrical length & resulting cm impedance,

All this stuff has been talked about modeled and posted on here several times with links to sites such as w8ji.com & kirchoffs current law.

 

[loosecannon]

ok, i looked back at my past posts in order to see why i got such a backlash to the info i posted.
my mistake was in terminology, so i'll take the blame for that part.

i never should have used the term "radiating" or "radiating element".
i was speaking of counterpoise, and have no idea why my brain chose those terms.
i did eventually start using the correct terms, but by then the damage had been done.

aside from that, it seems like the intent of my advice got completely lost because a couple of you decided to debate me instead of just correcting me.
yes, i should have worded my advice differently, but all i was trying to convey to the OP was that there is a chance that the choke might give him some confusing results if it was placed at the feedpoint of an A99 antenna, depending on how, and to what, the antenna was mounted.
admittedly i didnt do the greatest job of that.

while im admitting to starting the confusion, i would like to think that the others here, who were only to happy to point out my mistakes, might have been able to look at the context of the post and actually try to help stellasstillarat and keep this thread on track.

a simple, "i think you meant counterpoise", or "remember, if the antenna is mounted to a metal mast at least 9 feet long, then he would have no problems putting the choke at the feedpoint", would have sufficed.

lets try not to get too caught up in the "everybody look at how much i know, and how stupid you are" area of internet debating.
this thread is about stellasstillarat's antenna set up, and while i did make a mistake in my advice, it was given with the intention of helping the OP.

fourstringburn, and M0GVZ, ask yourselves how much help you were offering the OP with your posts to me.
Do what i did and go back and read your responses.
you will find that you left the OP out in the cold and instead concentrated all your efforts into making sure i knew how little you thought of my statements.

ive been studying antennas long enough to have forgotten plenty of things, and im sure i will make mistakes in the future, but my intentions will always be to help someone.
can you two say the same?
LC

 

[bob85]

vkrules,
That link you posted has me puzzled, it seems to contradict what i have seen in other peoples models which makes me think there could be something wrong with the models i have seen.

How i tackle no radial endfeds when friends have rfi issues is to,

add radials,
isolate the antenna from the mast,
add a choke at the feedpoint,

so far this has worked to get rid of rfi, give a small improvement in signal,
sometimes reduce noise from local noise sources.

 

[vkrules]

vkrules,
That link you posted has me puzzled, it seems to contradict what i have seen in other peoples models which makes me think there could be something wrong with the models i have seen.

How i tackle no radial endfeds when friends have rfi issues is to,

add radials,
isolate the antenna from the mast,
add a choke at the feedpoint,

so far this has worked to get rid of rfi, give a small improvement in signal,
sometimes reduce noise from local noise sources.

Thats how I get rid of the CMC issue too Bob with the same results you mention.. Sometimes just the choke works , sometimes the radials .Sometimes it takes both and isolate.
w8ji pretty much ,says the same thing .
Must say I've installed hundreds over the years and the end fed 1/2 wave always give me more trouble than end fed 5/8 wave.

 

[fourstringburn]

fourstringburn, and M0GVZ, ask yourselves how much help you were offering the OP with your posts to me.

How much were you offering by your admitted state of confusion?

Actually I been saying in my posts all along to use a choke balun at the source, the antenna feed point to reduce common mode in the first place. That's been the topic in this thread.

 

[loosecannon]

fourstringburn, and M0GVZ, ask yourselves how much help you were offering the OP with your posts to me.
Do what i did and go back and read your responses.
you will find that you left the OP out in the cold and instead concentrated all your efforts into making sure i knew how little you thought of my statements.

ive been studying antennas long enough to have forgotten plenty of things, and im sure i will make mistakes in the future, but my intentions will always be to help someone.
can you two say the same?
LC

nice try.
if you had quoted the whole statement; you would see that i already answered your question.
grow up and let it go dude. my previous post should have ended the argument.
if you don't feel like ending it, you can argue with yourself.
LC

 

[fourstringburn]

nice try.
if you had quoted the whole statement; you would see that i already answered your question.
grow up and let it go dude. my previous post should have ended the argument.
if you don't feel like ending it, you can argue with yourself.
LC

Glad you admitted your error, no hard feelings here!