Repair Imax 2000? Or build something?

[mr_fx]

I need a 10m vertical antenna for local work... I dont want to deal with radials...

I have an Imax 2000 sitting around... but the antenna is broken on the bottom section

 

[Robb]

The only commercially made antenna that doesn't need radials is either the Vector 4000 (inverted radials) or the GainMaster (vertical dipole w/tuning stub). Any other vertical antenna will need to have a counterpoise, even an Imax 2000. Otherwise, it will end up using the coax braid as the counterpoise with RF getting in the shack . . . Or build a loop antenna . . .

 

[vkrules]

home brew a gainmaster they work well

 

[Atlasta]

Is it broken where the second section connects to the base? That's a fairly common failure point on the IMAX.

If it is broken at that point and the copper rod inside isn't trashed you may be able to fix it by slipping a length of 3/4 " PVC pipe over the broken section, then applying a LOT of epoxy and sealant to either end. After that, just cut it down for 10m.

I've fixed a few of them this way.

 

[The DB]

The only commercially made antenna that doesn't need radials is either the Vector 4000 (inverted radials) or the GainMaster (vertical dipole w/tuning stub). Any other vertical antenna will need to have a counterpoise, even an Imax 2000. Otherwise, it will end up using the coax braid as the counterpoise with RF getting in the shack . . . Or build a loop antenna . . .

This isn't actually true. The only time you absolutely need radials is when you don't have some form of matching system, and if you have a 1/2 or a 5/8 wavelength antenna you will have such a matching system. Once you have such a matching system on the antenna, all you need is a very short conductive piece to cover the "other half" of the antenna, the matchign system does the rest. If you designed the matching system properly a two foot section of pipe such as is used on the a99 and imax antennas, or even less, will work just as well as a full 1/4 wavelength set of radials.

If you don't have radials then you need to be more careful of common mode currents. Radials do a very good job at keeping them from forming, but they are not the only way to control said currents.


The DB

 

[mr_fx]

The break on the imax 2000 starts right above the mounting bracket and goes 2 or 3 feet up...

 

[mr_fx]

I have a steel 2 car garage. I might try mounting a mobile 102 steel whip on it and use the garage as a counterpoise

 

[mr_fx]

I tried to take some rg58 cut off 8 feet of sleeve and roll the braid back over the sleeve... Then tied the center conductor to a rope and pulled it up into a tree...

Best match I found was like 2.9:1

 

[vkrules]

8 ft is to short ,try another 10 inches. then trim back or fold the top wire to tune ( top wire may end up shorter due to velocity factor of the insulation used,). Also wind a 5 turn 100mm choke just below the braid.( may not need both the braid and choke) to stop any cmc.

 

[vkrules]

Or just make a T2LT same thing but just cut off the braid, measure down about 102 inch then wind the coil. much easier than trying to pull the braid back over.

 

[Robb]

This isn't actually true. The only time you absolutely need radials is when you don't have some form of matching system, and if you have a 1/2 or a 5/8 wavelength antenna you will have such a matching system. Once you have such a matching system on the antenna, all you need is a very short conductive piece to cover the "other half" of the antenna, the matchign system does the rest. If you designed the matching system properly a two foot section of pipe such as is used on the a99 and imax antennas, or even less, will work just as well as a full 1/4 wavelength set of radials.

If you don't have radials then you need to be more careful of common mode currents. Radials do a very good job at keeping them from forming, but they are not the only way to control said currents.


The DB

We've had a similar conversation about the A99 before; remember that? Common mode current appeared despite the best means to tune it, and then adding radials as well as isolating it from the mast was the only way to get it to behave as one might expect of a internally matched antenna. I've had a similar experience with the Imax 2000 as well. Have you solved the dilemmas with your A99 antenna yet? I am curious because you never gave it your analysis; or did I miss it?

While these antennas are supposed to work w/o radals, their reality is quite the opposite. I agree with you that a properly designed antenna of this nature should work as you described; but what is offered commercially is quite unwieldy and contrary. Read 'cheap'.

What I offered as a simple solution that uses no radials; that is, to build a loop antenna. Enough gain and quiet if horizontally polarized, stealthy and cheap to build and maintain. If he wanted to get a commercial piece, I then suggest the GainMaster or the V4K. Clean designs w/o unsightly appendages. That is what I gathered from his question.

 

[The DB]

We've had a similar conversation about the A99 before; remember that? Common mode current appeared despite the best means to tune it, and then adding radials as well as isolating it from the mast was the only way to get it to behave as one might expect of a internally matched antenna. I've had a similar experience with the Imax 2000 as well. Have you solved the dilemmas with your A99 antenna yet? I am curious because you never gave it your analysis; or did I miss it?

You must be thinking of someone else. I don't own an a99 antenna and never have, although I have worked with my share of them over the years. I actually had a friends on my house at one point... Long story... I may have called it mine as it was on my house and I used it, but it technically wasn't. He didn't want it back, and I gave it away when I moved in to my current house and put up a Maco v5000.

I have analysed the a99 antenna pretty extensively using modeling including many setups that as far as I know don't actually exist anywhere in reality just to get an idea of what would happen. As far as gain is concerned, and once CMC's were dealt with, the differences with having radials in any configuration and no radials was so close you would never even notice the difference. Within this range of minor differences, a set of full length 1/4 wavelength horizontal radials actually had the least amount of gain, while a set of shortened angled radials that match the radial kit had the best gain.

I did mention that the main noticeable effect of using said radials was their ability to control common mode currents.

When it comes to the Imax, I also played with it in said modeling, and got a very different result. I didn't talk about it much on this forum though (or any forum for that matter). Both the models with radial same radial kit that you can buy with the shortened angled radials, and the model with full length angled radials, performed noticeably worse than anything with no radials or horizontal radials. The gain was there, but it was in a higher lobe. In this case, the downward angled radials added enough out of phase vertical polarization to affect the pattern of the antenna. The only Imax models that I have that performed worse than that is one with a 1/4 wavelength wire running straight down to simulate the suggestions of many people who say to put a choke that far away from the antenna for various reasons. A choke that far away from the antenna, any antenna, defeats the purpose of having a choke in the first place...

While these antennas are supposed to work w/o radals, their reality is quite the opposite. I agree with you that a properly designed antenna of this nature should work as you described; but what is offered commercially is quite unwieldy and contrary. Read 'cheap'.

And yet there are many people who add a radial kit to existing a99 antennas and see no noticeable difference, just like modeling predicts. As a matter of fact, this is the majority of reports that I am aware of. I agree that the a99's are cheaply made and could easily be made better, and I will always recommend another antenna, and by another antenna I mean any other antenna over an a99 or Imax antenna. That being said, if that is what someone has, I'll still work with it... I've had success with a ferrite based choke in combination with a two foot solid fiberglass rod as mast isolation for controlling CMC's.

What I offered as a simple solution that uses no radials; that is, to build a loop antenna. Enough gain and quiet if horizontally polarized, stealthy and cheap to build and maintain. If he wanted to get a commercial piece, I then suggest the GainMaster or the V4K. Clean designs w/o unsightly appendages. That is what I gathered from his question.

Fair enough. In that regard, if there is a tall tree that he can get a wire up pretty far to hoist a wire antenna, a T2LT antenna will also work well, and be a cheap alternative. It could also be mounted horizontally if that is what is desired.


The DB

 

[mr_fx]

8 ft is to short ,try another 10 inches. then trim back or fold the top wire to tune ( top wire may end up shorter due to velocity factor of the insulation used,). Also wind a 5 turn 100mm choke just below the braid.( may not need both the braid and choke) to stop any cmc.

I folded the braid back... IE the coax runs inside the braid.... The braid goes down 8ish feet... The center conductor goes up 8ish feet

In total its around 16 feet

 

[Robb]

You must be thinking of someone else. I don't own an a99 antenna and never have, although I have worked with my share of them over the years. I actually had a friends on my house at one point... Long story... I may have called it mine as it was on my house and I used it, but it technically wasn't. He didn't want it back, and I gave it away when I moved in to my current house and put up a Maco v5000.

I have analysed the a99 antenna pretty extensively using modeling including many setups that as far as I know don't actually exist anywhere in reality just to get an idea of what would happen. As far as gain is concerned, and once CMC's were dealt with, the differences with having radials in any configuration and no radials was so close you would never even notice the difference. Within this range of minor differences, a set of full length 1/4 wavelength horizontal radials actually had the least amount of gain, while a set of shortened angled radials that match the radial kit had the best gain.

I did mention that the main noticeable effect of using said radials was their ability to control common mode currents.

When it comes to the Imax, I also played with it in said modeling, and got a very different result. I didn't talk about it much on this forum though (or any forum for that matter). Both the models with radial same radial kit that you can buy with the shortened angled radials, and the model with full length angled radials, performed noticeably worse than anything with no radials or horizontal radials. The gain was there, but it was in a higher lobe. In this case, the downward angled radials added enough out of phase vertical polarization to affect the pattern of the antenna. The only Imax models that I have that performed worse than that is one with a 1/4 wavelength wire running straight down to simulate the suggestions of many people who say to put a choke that far away from the antenna for various reasons. A choke that far away from the antenna, any antenna, defeats the purpose of having a choke in the first place...



And yet there are many people who add a radial kit to existing a99 antennas and see no noticeable difference, just like modeling predicts. As a matter of fact, this is the majority of reports that I am aware of. I agree that the a99's are cheaply made and could easily be made better, and I will always recommend another antenna, and by another antenna I mean any other antenna over an a99 or Imax antenna. That being said, if that is what someone has, I'll still work with it... I've had success with a ferrite based choke in combination with a two foot solid fiberglass rod as mast isolation for controlling CMC's.



Fair enough. In that regard, if there is a tall tree that he can get a wire up pretty far to hoist a wire antenna, a T2LT antenna will also work well, and be a cheap alternative. It could also be mounted horizontally if that is what is desired.


The DB

Well; yeah. It was never about the gain, it was about having to control the CMC. I imagine that having installed radials on either one of these antennas and then adding the a choke coil for good measure should give a slightly better polar pattern as well.

Is that what you found?

Back on subject . . .

mr_fx:
Try a loop. I convinced a local around here to build a loop antenna. He used an antenna tuner on it to make it work. But it can be done w/o a tuner by making a very simple matching section out of a piece of common 75 ohm cable TV coax approximately 5 feet 10 1/2 inches long. The length of the loop wire is 36 ft 4 inches to be centered on 27.555mhz.

The results were dramatically different to my station that his homebrew 1/4 wave antenna had, as he lived in a small valley. He also had it up about 25 ft suspended on some cheap PVC tubing. Worked a treat. They are quiet and have decent gain. If you want to change the polarity from horizontal to vertical, just feed the coax to one of the upper corners rather than at the base. But for a cheap, effective DX antenna, it is the best in its class - IMO . . .

BTW, the length of your dipole is wrong, need to be close to 9 ft on each side. If it is set up vertically; then be sure that the feed point of the coax is at a 90 degree angle away/perpendicular from the dipole for at length for at least 6 ft.

Keep whichever antenna you build at least 18 ft above that metal roof, as the metal will interact with the roof - unless it is a 1/4 wave vertical as that should work. Not much of an effective height that way, but - oh well . . .

 

[mr_fx]

The metal roof is another structure not connected to my shack. Only mentioned the roof because I was considering using my 102 whip on the roof of the two car garage that is not too terribly far from my shack. I am familiar with the loop but this is going to be for working locals for working DX I have my three element tri band