A99 VS. I MAX CHOICE AT IMPROPER HIGHT

[Stellasstillarat]

Thanks for taking the time to read what was most likely to many word's to exsplan why I will not be racking my brain on who's choke design I (turns diameter) should go with.

The idea of having to coil, measure, wiretie then fasten the finished choke to the mast sevral times before I get it right (if I manage to get it right at all) is, disparaging at best.

I haven't read g3gxg's article yet but will emidietly upon posting this reply.
I suspect his choke design will vary to a certain degree of all the others who have tested several design's and not only disagreed with all the design's created by the other mathamatical antenna geniuses that already have determined the correct amount an 11 meter (27 mhz) choke (ugly balun) turns and diameter it positively unequivocally 100% should be.

Then posted his perfect design for anyone who needs to build a choke the way it should be built and the exact placement of this choke below the feed point, 24"'s, 21, 18 and 3/4:'s. It's frustrating because I want to do it right the first time.
Truth be told If I had to try it a couple of times in order to get it right, great!

This reminds me of a qso going down on 38 lsb about using 18' of coax between your rig and Wilson mag mount, then someone qsk's and says "no it's 17'. Then two hours later someone says those coax mesurments when installing a setup in your auto are bs. That subject like most has been beaten with a stick so many times and even though it's been established how much length of cable should be used is the amount that's needed to reach from you Wilson to Your radio is the correct amount (according to the last article I read).

I'm not intested in the exact length of coax needed in that situation because I have no interest in installing a mobile rig.

Now the choke is something that's gonna make my antenna perform the best it could and is something I'm totally convinced of, but it's the design of it for LMR 400 (or any particular size coax) for use on 27 mhz that's driving me up a tree.
I know I'll eventually be convinced by one or more of the authors and designers that his is the correct way one should prepare and where below the feed point it should it be placed.
That has yet to happen. As a matter of fact the only thing I've gotten out of all the posts and links to sites describing the correct turns and dimensions of a cb radio (ugly balun) choke using LMR 400 is confused.

Read the full page.

http://www.karinya.net/g3txq/chokes/

Steve G3TXQ has done a lot of experimentation into baluns, ununs and common mode RF. The chart I posted is one he did using proper measurement techniques and equipment. As far as I know he's the only one who has published such data.



It is a necessity. Whenever you're using an unbalanced feeder, aka coax, you need one to prevent the coax becoming part of the antenna even when you're using a perfectly balanced dipole antenna.
The problem with it is that in many cases there will be no noticeable difference to the end user so they think that its doing nothing because noise on RX and RFI is the only measurement they use and that was the same after they installed a RF choke as before they did. They don't ever measure common mode on the coax and, in the case of balanced antennas like yagis and dipoles, the radiation pattern as it is altered by not having a RF choke and the coax becoming part of the antenna.

Without a RF choke throw 100W into your antenna, get a small flourescent tube and place it near the coax at various points and you'll see it light to various degrees with no other source of power. That is proof of the existence of common mode. Fit your RF choke at the feedpoint and repeat the experiment. Depending on the effectiveness of the choking and assuming its been built properly you'll see either the tube not lighting at all or lighting far dimmer thus proving the choking is working.

If it helps, think of using a RF choke as "best practice". Do it for the same reasons you don't just take your coax, strip back the insulation and just shove the wire into the antenna socket, holding it in place with matches and insulation tape and instead solder a PL259 on the end even though the other way works.

 

[wavrider]

http://www.eham.net/reviews/detail/7780

https://forums.qrz.com/showthread.php?311797-MFJ-915-Line-Isolator-vs-a-regular-balun

http://www.gigaparts.com/Product-Lines/Baluns-Ununs/MFJ-915.html?gclid=CIXHwrzD4MYCFQqSaQod1K0Omg

There you go Stella, end all the debates of RF choke and winding coax

 

[Stellasstillarat]

I'm fairly positive there will be postings saying either just because it's made by MFj it's junk or the design of this type of choke doesn't work.

I say this because no one until yourself ever brought this item up.

I read the reviews and for $27 and change (gigaparts) it should be on it's way to me by Monday. Thank you so much.
I'm blown away. I believe I never came across the MFj 915 or the like because i always searched using the words like rf choke and ugly balun. I can't thank you enough. I will post report's of the readings without it inline then inline.
I just recived my MFj 1500 watt dummy load, 828 digital /cross needle meter,Dawia antenna /transceiver switch and and a brand new a99 as I'm not taking any chances.

I also Changed the pl 259's on my 100' LMR 400 and eight LMR 240 jumpers as I couldn't track down the problem and feel with my lack of knowledge it's gonna be cheaper in the long run, money and time wise. I'm confident I will be back up and on the air by next week end.
Wavrider. Not only was it your words of insprration that stopped me from packing everything up and putting them away.

Now this MFj 915 that may work as well as any coax Built choke but for some who lacks the knowledge and was becoming confused by all the different postings on the way it should be constructed for under.$40 delivered imo this can't be beat. Thanks for taking the time to add the links and your grasous help. Good looking out. Optimisim is sneaking into my pessimistic shell. Thanks to everyone who weighted in.

http://www.eham.net/reviews/detail/7780

https://forums.qrz.com/showthread.php?311797-MFJ-915-Line-Isolator-vs-a-regular-balun

http://www.gigaparts.com/Product-Lines/Baluns-Ununs/MFJ-915.html?gclid=CIXHwrzD4MYCFQqSaQod1K0Omg

There you go Stella, end all the debates of RF choke and winding coax

 

[wavrider]

No problem Stella.

If one doe snot know what to look for on CMC besides the obvious, TVI, RFI, PC speakers etc etc then it will go un-noticed.

The First issue I was able to say definitely that a 1:1 current balun workd to solve CMC was on a 40 meter ground mounted phased array,.

I was trying/learning about the Christman method of forced current phasing and one antenna was not playing right.

After a few days of scratching my head I decided to put a 1:1 current balun at the feed point of the one vertical, Finally the array started acting like it was supposed to.

It was a fun learning experience and made a believer out of me when it comes to chokes and baluns

 

[Stellasstillarat]

As I was curious about the MFj 915 I purchased I thought I'd download the instruction manual.
It is a one page instructional exslaning the install. It directs the new owner to install it at the point of tx.
Which I thought to mean at the output of the transceivers so 239.
It didn't make sense as I thought the choke was to "choke" the rf from traveling down the antennas feed point and back into the shack.
So I began to do some more searching. And sure enough came upon a wwdx posting from 2008 saying the MFj 915 instuctions install manual is incorrect.
Not many replyed to the op but those that did said MFj is known for printing improper manuals for many of there items but normaly would correct them in time.
I would think if they corected it seven years later they also would have corrected the free downloadable manual but they haven't.
If I'm going to get the same instruction manual from 08 I'm obviously gonna ignore it. Im assuming a jumper will be installed in the antennas so 239 then my lmr 400 from the transceiver screws into the other end of the 915.
If I'm correct should the 915 be placed 2' below the feed point as if it were treated as an ugly balun (coax choke?)
If anyone uses an mfj 915 or has ever insalled one, can you please tell me the proper way to install it? Id hate to wait for the day I recive it to begin asking how to correctly put it on the a99. Im kinda hoping MFj updated their single .page three paragraph instructional manual. But it's unlikely. Thanks. P.s. I was very impressed with just about every review posted on the 915.many differnt people use the 915 on many bands and almost every one was very impresses with it's performance at It's low price. Im confident it will work as well as if I had spent two days trying to get the correct turns and dimensions for a home brew choke. And truthfuy l probably never would've got it correct. QUOTE="wavrider, post: 526486, member: 9893"]No problem Stella.

If one doe snot know what to look for on CMC besides the obvious, TVI, RFI, PC speakers etc etc then it will go un-noticed.

The First issue I was able to say definitely that a 1:1 current balun workd to solve CMC was on a 40 meter ground mounted phased array,.

I was trying/learning about the Christman method of forced current phasing and one antenna was not playing right.

After a few days of scratching my head I decided to put a 1:1 current balun at the feed point of the one vertical, Finally the array started acting like it was supposed to.

It was a fun learning experience and made a believer out of me when it comes to chokes and baluns[/QUOTE]

 

[Heavy Metal]

You do not want to add the beads into the 5 turns, it'll skew how it works, basically what frequency its tuned for, because it is an LC circuit. By adding beads you're altering the overall inductance and it'll affect the capacitance because you're also altering the spacing between the turns so therefore the capacitance between turns.

Think of a RF choke being similar to a tuned trap LC circuit for a multiband single element antenna. Alter either the capacitance or the inductance and you shift the frequency it works at because the frequency it works at is dictated by the formula

1/ 2pi√LC


What would be better is adding them to the feedline prior or after the air wound choke because they have no effect on the LC circuit. So assuming its correct as per the chart when you measure it, the choking of the two is combined so if you've 2k from the air wound choke at a certain frequency and a total of 1k from the beads ( the spec sheet should have a chart for impedance at specific frequencies for numbers of turns), you've a total of 3k at that frequency.

Gotcha...


Now finally which is better to use? For single banded/wide banded vs multi banded. And why would it be beer than the other for stray RF and internal / external interference.

 

[Stellasstillarat]

Wavrider. With some extra research I found out that the MFj 915 isn't a replacement for an air choke.
With that being said, if I have a problem with the workings of the new a99.
I will install my 11 meter wire dipole.

The mfj 915 is a line choke is for such an antenna (dipole) and will keep it if I go with the dipole.

I'm actually thinking about putting up the dipole in a horizontal configuration even if the new a99 and everythiing else I'm replacing works.

I wasn't able to track down the cause of what was causing my serving ref and lowering of ny output when going higher in frequency. I did everything possible to find the problem by eliminating what could've been the cause. I did this by removing every apliance one by one but could track it down. I Started by replacing jumpers one by one. Then removed my maco 150 preamp.Then the amplifier until i was left with the madison, meter and antenna. With no change in the problem. I then replaced the madison with the Washington. Still no change. Next I took the antenna out of line and installed a 20 watt 60 watt pep dummy load to the meter and Wallah! The swr was not only perfect but the output dead key was 4 watts with 10 watts pep on channel 1 - 40 with a negligible change in swr and output throughout all 40 channels.

So I assumed the problem had to be in the antenna system somewhere between my coax and antenna itself. I even removed the Madison and replaced it with the Washington just to dubble check the dummy load readings and I got the same dummy load results with the Washington.rigs arent the problem.

Getting back to The 915. I may or may not need it.
the seller whom I purchased it from does have a 30 day no questions asked money back return policy.
I purchased the MFj 868 and a 1500 watt dummy load off her so she's gonna actually extend it to a 60 day money back return.

I'm assuming why no one mentioned the 915 for a replacement for air wound rf choke aka ugly balun. Is because there two different items.

I've read that one of the reasons an air choke works as it does is there's no breaks in the air choke. If the mfj was installed at the antenna feed point it would need to be installed with a jumper causing a break in what should be a continuss loop on a single piece of coax.

I'm not sure if the articles I read are correct or even if I'm understanding or exsplaning what I read properly.
If not can you or someone exsplan what the MFj 915 's pupose is for?

I also wanna say, wavrider wheather I'm correct or not I have no ill felling for your suggestions and please continue to answer and give advice on any questions I ask as I value your input and knowlage.

I also found through my reading that LMR 400 although an excellent coax has an inherent problem due to it's ridgitness and if bent especially at the ends where the pl 259's are installed can actually kink and break the solid piece center soid copper wire.

If I'd done more research and asked more questions before i purchased the 400 I would've purchased the ultra flex instead or the plastic like outer sheild the LMR 400 has. if i do and im most likly gonna put the dipole up for skip use which i beleve wil l peform better than the a99 for dx'ing. I will purchase the lmr 400 or maybe the 240 for use on the dipole..

I've left nothing to chance on the new install. I did change the pl 259's on my 100' LMR after I cut the extra coax off that just wasn't needed (about 25'.

I did a very poor job of relacing the pl 259 where the extra 25' was cut, I did agan. I'm not sure the second 259 i installed was any better than the first one.
This alone would be enough to cause the problems I was exsperiancing

I sent my eight lmr 240 and four 400 jumpers, the 100" transciver to antenna LMR 400 to my tec to test and change out any 259's that are problematic and i suspect he will find more than a few bad ones. I'm confident everythings gonna work fine with the new insall. F I have a decent working station but know I can get it better i will concider purchesing a rohn or the like telescopic mast as to get the antenna above the roof line. What bothers me is at about 12' above the ground my station worked fine until I added another 5' mast. And even though I removed it the 5' mast the problem remained. I'm very optimistic beginning with the new a99 and any pl 259's replaced will correct the problemd anf I'll be back on the sir enjoying my hobby for many years to come. Thank to everyone who's taken the time to help me every time I ask fot help.

http://www.eham.net/reviews/detail/7780

https://forums.qrz.com/showthread.php?311797-MFJ-915-Line-Isolator-vs-a-regular-balun

http://www.gigaparts.com/Product-Lines/Baluns-Ununs/MFJ-915.html?gclid=CIXHwrzD4MYCFQqSaQod1K0Omg

There you go Stella, end all the debates of RF choke and winding coax

 

[Oatmeal]

Some of my locals runs a A99, and they say its better than the Imax 2k, can't say anything about the A99, I myself don't like the fiber glass antennas, but this Imax has surprised me....used it for years on the 11m band, just here the last few months started using it on 10m, and it has done a fine job for me....it's just 21ft off the ground.....

 

[The DB]

Wavrider. With some extra research I found out that the MFj 915 isn't a replacement for an air choke.
With that being said, if I have a problem with the workings of the new a99.
I will install my 11 meter wire dipole.

The mfj 915 is a line choke is for such an antenna (dipole) and will keep it if I go with the dipole.

...

I'm assuming why no one mentioned the 915 for a replacement for air wound rf choke aka ugly balun. Is because there two different items.

I've read that one of the reasons an air choke works as it does is there's no breaks in the air choke. If the mfj was installed at the antenna feed point it would need to be installed with a jumper causing a break in what should be a continuss loop on a single piece of coax.

I'm not sure if the articles I read are correct or even if I'm understanding or exsplaning what I read properly.
If not can you or someone exsplan what the MFj 915 's pupose is for?

I'm not sure what you have or have not read on the subject, but an air choke is, at best, a third rate attempt at replacing the functionality of a current balun. A current balun can be used on any antenna, and it has the effect of fully isolating and removing common mode currents, among other benefits. A choke, on the other hand, simply attempts to attenuate said common mode currents. In the case of the choke some CMCs always make it through.

Also, a well made current balun works over a much wider frequency range than even a ferrite based RF choke, while an air choke that was made wrong (intentionally or otherwise) can actually make a common mode currents issue worse on some parts of the frequency range (hopefully not the part you are using it on).

Now here is the bad news, the MFJ-915 is not actually the current balun they say it is...

Its made up of 50 Ferrite Core Beads, placed onto a 13-inch piece of RG-303 coax.

Taken from http://www.mfjenterprises.com/Product.php?productid=MFJ-915. What this is telling us is it actually is nothing more than a ferrite based RF choke, and because of this it is actually nothing more than a more advanced version of an air wound choke. By more advanced I mean it is more resistive instead of reactive in nature, which has certain advantages. Plus you can make one yourself without having to worry about the break in the coax stuff, just by some FB-31-1020 beads, 10 will work well, and string them on the coax before you put an end on, and then find something that will hold them in place near the antenna end of the feedline (I personally use heat shrink tubing for that).

Now about that break in the coax business. I'm sorry to have to say this, but what you have learned about this is a load of crap. Yes, there is an amount of loss going through such a connection, but it is insignificant to the point of being well beyond irrelevant. To put this loss in perspective, my AIM 4170 VNA can barely measure the difference a single PL-259 connection has. These connections have so little loss that I had to string a bunch of them together to get a meaningful measurement and make any useful calculations. You can have well more than 20 of them on your run and (assuming they are all properly connected) you won't notice the difference.

If we were talking UHF frequencies, then yea, these connectors would be a problem, but we aren't talking about UHF frequencies here are we...


The DB

 

[Stellasstillarat]

I purchased a 2000 with every intention of replacing it for the a99. Now I know many will say my reasoning for getting another a99 to replace the old one which may or may not have had a problem is this.
The a99 is infamous for causing rfi, now imagine that a99 at only 12' above the ground, rfi should be a given, but after nine months of use not a single complaint from nabour's about interferance.
And the house which it's closest to is of course my own, and no rfi coming through a single appliance. .
The I max although not as infamous for rfi compaired to the a99 still has its rfi issues.
My fear was changing to the 2000 may cause rfi that the a99 has atleast for me doesn't exist.
Is the 2000 a better antenna? of cause.
It's a 5/8 wave opposed to a 1/4 wave, but the chance the 2000 may cause me an issue that the a99 doesn't is for me worth the lesser performance. Stranger things when concerning antennas have and do happen every day.
And that's all I have to say about that.

Some of my locals runs a A99, and they say its better than the Imax 2k, can't say anything about the A99, I myself don't like the fiber glass antennas, but this Imax has surprised me....used it for years on the 11m band, just here the last few months started using it on 10m, and it has done a fine job for me....it's just 21ft off the ground.....

 

[Oatmeal]

My Imax is out back, sets in the corner of my porch and is 12ft from the power lines, and my living room wall is on the other side of my antenna, and no problems with it messing anything up..but I'm not running alot of power either, just a hf radio and 30 watts....and it's a few ft from our dish antenna, no choke on it, nothing....using RG 213 coax, and some LMR 240 coax on it, thats all.....good as it was talkiing on 10m can't complain with it.......

I have read a bunch of stories on the A99 and the Imax 2k....

My neighbor down the road runs a A99 and he has problems running a HP radio on his messing up his neighbors stuff...he talks late at night now, lol..

 

[M0GVZ]

Gotcha...


Now finally which is better to use? For single banded/wide banded vs multi banded. And why would it be beer than the other for stray RF and internal / external interference.

Single wide banded is always one with a ferrite. Narrow banded is possibly the air wound.

However the ferrite based one is going to be better 9/10. First of all its choking is mostly resistive - if you've read the G3TXQ page he mentions how reactive choking can actually increase common mode making it worse. Secondly its more tolerant of discrepancies in the building due to the wide almost 20MHz bandwidth of it. Because of how narrow banded the air wound one is, the only real way to guarantee that an air wound one is going to be choking where you want is to use a GDO to measure the resonance of the choke you've built.

 

[M0GVZ]

but this Imax has surprised me....used it for years on the 11m band, just here the last few months started using it on 10m, and it has done a fine job for me....it's just 21ft off the ground.....

It should tune up on 12m and 15m as well without needing to alter the tuning rings and probably without even needing a tuner other than to trim a slightly high SWR but mine was below 2:1 as I recall. Some have reported getting it working on 17m with an internal ATU but when I tried it with mine it wasn't any good.

 

[wavrider]

Stella DB answered your questions, and YES it will suit your purpose of trying to prevent CMC on your coax and allow you to get accurate measurements of your antenna system.

Line isolater, RF choke, 1:1 current balun all do about the same.

Exception to a TRUE current balun which forces equal currents onto a balanced antenna using an unbalanced feed line.

So Stella you are using coax, unbalanced feedline, and a vertical antenna, unbalanced antenna,so either the RF choke or Line isolator will suit your needs where a 1:1 current Balun will not.

 

[Needle Bender]

Stell, Add about 6-8 21" radials to the A99 at the base just below the tuning rings, then mount it on a 6' x 1"-1.5" fiberglass rod and wrap 5 1/2 turns of your coax in a neat coil about 4" inside diameter around the fiberglass rod about 6" below the mounting tube down the fiberglass. This will both decouple RF from the coax and supply just the right amount of counterpoise for a bottom-fed 1/2 wave, though you might want to try adding 12"-18" to the top of the A99 to bring it up to 17.5" for a full 1/2 wave above the tuning rings. This should give both the best performance and best isolation from RFI.