Should i use a homemade CMC on my New antenna setup 100 feet up a tree?

[jtrouter]

Ok i am now about two weeks until my tree climber is going up my 100 plus foot tree to mount my antenna.
I have never used a CMC on a base antenna before however i am much more read then i have ever been.
So As of now my antenna choice #1 will be a original Avanti Astroplane if he installer thinks he can get it up the tree
and if he can mount it so the bottom of the hoop clears the top of the tree WITHOUT topping it. Plan 2 is for me to
have a new Imax ready in the box if needed. I did find a new in the box Max2000 however i will not know if its a 24footer
or less until i open it. Now for the questions I understand how to wrap around a 4 inch form to make a CMC should be done
but with the Astroplane I assume it should be placed Below the hoop? how far below the hoop? and if not where?
If i have to use the Imax my understanding is that without the groundplane kit it uses the coax as a counterpoise? or part of the ground?
Should i use a CMC with the IMax? Also if it matters the coax is LMR400 and near the bottom we are putting a lightning arrestor/Tap for the wire to
the ground rods would or should i put the CMC at that location? Ans because LMR400 is hard to bend i plan on using a much softer piece of coax
to make the CMC would coax would you recommend for doing this? Thank you all for reading and im sorry that i have so many questions but i only have
the climber coming for one day i cant afford to pay him to come back. What to get this right the FIRST and only time.

 

[Alabama Buckeye]

What is a CMC?

The Imax 2000, I guarantee is not 24". $50 bet. No such thing! Never was!
At best it was regarded as a .64 wave antenna, never .666...6. Closer to 23 than 24'

A 100' of LMR400 dangling vertical from the feed point! Your are employing a tension relief measure? Maybe 1/16" wire rope and ferrules, or at least 3/16" Dacron Line.

 

[Alabama Buckeye]

On your original previous posting on this matter I had inquiry principally pertaining to Imax. But your tree climber's concern for the wellness of the other antenna was valid, IMO. Again, good luck.

There are a few question to answer; perhaps avoiding trouble of your own making:
How are you attaching to tree?
What is your ground plane?

 

[nbsr]

Here's an interesting read on ferrite vs coax loops.
Not trying to start an argument but interesting non-the-less.

 

[Wire Weasel]

Me too .... what is a CMC ?

 

[AudioShockwav]

A common mode choke.
Often a air wound choke made from coax.

73
Jeff

 

[unit_399]

I use a common mode choke on every base antenna. Whether it calls for it or not. Better safe than sorry.
- 399

 

[loosecannon]

if you have 9 feet of metal mast coming down past the hoop, you can just chokes the coax at the feedpoint.

if you don't, then put the chokes 9 feet down past the hoop.

make sure you run the coax right down the center of the antenna and straight down for at least 9 feet below the hoop.

also, using snap on ferrite chokes is superior to using the coiled coax method.
you can get them from palomar engineers for about 40 bucks.
they come as a set of 5 or 6 chokes sized for your particular coax.

when you run the coax in to the house, you MUST add a ground rod and a real lightning arrestor right before the coax goes inside.

This ground rod needs to be tied in to your main house ground rod and any other ground rods you have in your system.

you should also ground all of your equipment to this same ground rod that is right outside the shack.

try to avoid using lengths between 7-11 feet when grounding all of your equipment to a common grounding point.
LC

 

[jtrouter]

So when looking at Palomar's web site i found this item that i wll attach.
I this better or worsre than just using a couple of snap on ferrites?
And if so where would i put the couple of snap ons? The one in the picture looks to be the best of both worlds?

 

[Alabama Buckeye]

So when looking at Palomar's web site i found this item that i wll attach.
I this better or worsre than just using a couple of snap on ferrites?
And if so where would i put the couple of snap ons? The one in the picture looks to be the best of both worlds?

"CMC", oh sure I've heard of 'em, just not by the acronym alone. I call 'em Fugly Baluns, principally due to size and weight attributed to the wound coax. I've adopted the FT240-43 wound alternative; a much smaller package.

I don't know how many snaps ons are the recommended for 27 MHz. I feel it's considerably more than a few; and, when it come to the in shack end of each coax there are 8-10 of 'em on mine.

 

[Marconi]

I don't know how many snaps ons are the recommended for 27 MHz. I feel it's considerably more than a few; and, when it come to the in shack end of each coax there are 8-10 of 'em on mine.

Bob85, a WWDX senior member, use to talk about CMC problems and how proper chocking helped his station communicate long distances.

I would ask him what he did and how he made his CMC solutions, and he would just say...what I do here will not work for you...every situation is different. Hard to disagree with that answer.

I can see this problem with some of my Eznec models, this bad CMC effect (feedline RF) that often skews the antenna pattern thus resulting in added losses, lower gain, and high angled lobe patterns.

Can you tell us how you determine these effects...when working or not?

 

[unit_399]

if you have 9 feet of metal mast coming down past the hoop, you can just chokes the coax at the feedpoint.

if you don't, then put the chokes 9 feet down past the hoop.

make sure you run the coax right down the center of the antenna and straight down for at least 9 feet below the hoop.

Matt -
Can you please give me an explanation for the above ?? As much detail as possible. I need to get a better understanding of how common mode currents work. Figured that I'd start here.

Thanks, mi Amigo.

- 399

 

[loosecannon]

sure thing 399!

it's not really about the CMC's themselves, but more about how this antenna works.

it uses either the metal mast its mounted on, or the coax shield itself as a sort of "counterpoise" (i use that term loosely) to aim the take-off angle.
by looking at the patent sheet, you can actually modify this antenna to aim downwards, upwards, and in between.

so you need to have a straight length of metal going straight down below the hoop for 1/4 wavelength for the antenna to work properly.

i actually shouldn't have said that you can put the ferrite right up at the feedpoint, because the CMC's will just induce right back into it because it runs right down the mast.

so you don't want to choke off the CMC's until after that 9 foot point so that the counterpoise works properly.
doing this forces the currents to recirculate in the antenna instead of sending them down the shield of the coax into your shack.

i always have to warn people about putting antennas in trees where the mast doesn't reach the ground and get grounded there.
if you don't put in a lightning arrestor right outside the shack, you could walk in there and see a sparking radio should a lightning storm come through.
even sand and dust blowing in the wind can create static on your antenna!

my last install of one of these antennas, i used a fiberglass mast, and used the coax as the counterpoise on purpose, and choked off the CMCs 9 feet down from the hoop.
coax grounded right before it entered the house.

this way, im not dealing with capacitive losses by having my antenna grounded, and it was basically isolated up in the air.
it. worked. amazingly!!!

then i had to move...
hope this helps.
LC

 

[jtrouter]

sure thing 399!

it's not really about the CMC's themselves, but more about how this antenna works.

it uses either the metal mast its mounted on, or the coax shield itself as a sort of "counterpoise" (i use that term loosely) to aim the take-off angle.
by looking at the patent sheet, you can actually modify this antenna to aim downwards, upwards, and in between.

so you need to have a straight length of metal going straight down below the hoop for 1/4 wavelength for the antenna to work properly.

i actually shouldn't have said that you can put the ferrite right up at the feedpoint, because the CMC's will just induce right back into it because it runs right down the mast.

so you don't want to choke off the CMC's until after that 9 foot point so that the counterpoise works properly.
doing this forces the currents to recirculate in the antenna instead of sending them down the shield of the coax into your shack.

i always have to warn people about putting antennas in trees where the mast doesn't reach the ground and get grounded there.
if you don't put in a lightning arrestor right outside the shack, you could walk in there and see a sparking radio should a lightning storm come through.
even sand and dust blowing in the wind can create static on your antenna!

my last install of one of these antennas, i used a fiberglass mast, and used the coax as the counterpoise on purpose, and choked off the CMCs 9 feet down from the hoop.
coax grounded right before it entered the house.

this way, im not dealing with capacitive losses by having my antenna grounded, and it was basically isolated up in the air.
it. worked. amazingly!!!

then i had to move...
hope this helps.
LC

Ok so to make sure i understand what your saying, I have my astroplane mounted on a 20 foot mast and it will be mounted so that he hoop will be a few feet above the top of the tree. So the plan should be to mount the CMC 9 feet or more down from the bottom of the hoop. From CMC down the 100feet plus at the bottom of the tree I will have a lightning arrestor with a couple of 6 to 8 foot copper ground rounds depending on how deep i can pound them as a hundred or more years ago a creek ran thru this area, So who knows how deep i can go without hitting rocks. From there the LMR400 will be buried then run into the house. Good plan?

 

[loosecannon]

Ok so to make sure i understand what your saying, I have my astroplane mounted on a 20 foot mast and it will be mounted so that he hoop will be a few feet above the top of the tree. So the plan should be to mount the CMC 9 feet or more down from the bottom of the hoop. From CMC down the 100feet plus at the bottom of the tree I will have a lightning arrestor with a couple of 6 to 8 foot copper ground rounds depending on how deep i can pound them as a hundred or more years ago a creek ran thru this area, So who knows how deep i can go without hitting rocks. From there the LMR400 will be buried then run into the house. Good plan?

yes good plan.

however if you have mast running down past your chokes and the coax runs right next to it, those CMCs will just induce right back into the coax after your chokes.

the amount might be negligible but you might as well just place your chokes on the coax right where the bottom of the mast ends up. no big difference between putting them exactly 9 feet down or 20 feet down.

make sure to use a lightning arrestor that costs upwards of 50 to 100 dollars.
the kind that comes with a replaceable gas charged cylinder. you don't want the kind that is really just a barrel connector with a lug for a small ground wire.
those things are junk and won't protect you.

it's best to place the lightning arrestor right outside the shack where the cable comes in, but all installations must make compromises.

you do need to make sure you tie in your ground rods with the main ground rod for your house electrical system. this is NEC code and if you don't, your insurance company could deny any claim you put in for damage from lightning and such.

as for the antenna itself, i'll give you a couple of tips.
seal the antenna where you screw in the top vertical section.
water likes to get in there.

also the hoop likes to sway a LOT in the wind and will mess with your SWR and take off angle because the mast being perfectly centered inside the hoop is part of the design. (lots to be learned from reading the patent sheet for this antenna!)

so it's best to find a way to "guy" the hoop around the mast so it stays put.
HOWEVER!
you don't want it too rigid. so no cable ties, and dont just use rope.
get some thin UV rated bungee cord and use that, tightening just enough so that if you really try, you can move the hoop around a bit, but in general it wants to stay centered in the circle.

hope this helps!
LC