Lets logically end the LMR-400 debate.

[Crawdad7]

New Newbie Build.

So when I attach the LMR400 to the MAXX2000 I will use a midpoint ground system to ground Coax to Mast, then go 18-24" down from antenna connection and do a 8 turn 8" Choke.
So I make this first assembly & null this out based on every other 1/2 Wave measurement.
So first line is approx. 38' + 2' for choke =40'
1/2Wave on 27.205 is 15.37'.
So this section will need to be roughly 47' & start cutting till nulled out.
Correct?

Then we hit a poly phaser in an enclosure before we go into garage.

 

[brandon7861]

New Newbie Build.

So when I attach the LMR400 to the MAXX2000 I will use a midpoint ground system to ground Coax to Mast, then go 18-24" down from antenna connection and do a 8 turn 8" Choke.
So I make this first assembly & null this out based on every other 1/2 Wave measurement.
So first line is approx. 38' + 2' for choke =40'
1/2Wave on 27.205 is 15.37'.
So this section will need to be roughly 47' & start cutting till nulled out.
Correct?

Then we hit a poly phaser in an enclosure before we go into garage.

What I said was more of a general theory rather than exactly what to expect. There are too many variables to say with any certainty. Look how a parasitic element can change the feed point impedance of an antenna. Structures near the coax will do the same. I merely brought it up as a partial explanation for a lack or presence of common-mode current.

If you are going to use a choke, why bother with any of this??

Although I have not tried yet, if I absolutely had to manipulate the common-mode impedance by altering the coax length, I would probably use the vertical as a counterpoise and connect the VNA to the common-mode path (but it would take a custom connector to disconnect the center conductor from the antenna so it wasn't being driven from above). That should allow me to make adjustments to the coax watching for a high impedance point.

Regarding your lengths, if the polyphasor is grounding the coax shield, then we are talking about odd multiples of quarter wavelenths as a ground a half wavelenth away will appear as the same low impedance at the beginning of the common-mode path. I also think that using the velocity factor for the inside of the coax will be an issue. The velocity factor on the outside is much closer to that of the VF in air. Dielectric constants and the associated formulas for VF, the other conductor needs to be in close proximity confining the signal in that material for it's dielectric constant to matter. Trim a dipole made of 14ga wire with PVC or THHN coating. Its VF above 95%,. Just put a choke on it ad call it a day.

 

[Crawdad7]

Ok so change of plans (because it is a Maxx 2000
NO midpoint ground of Coax.
NO PolyPhaser.

Choke or No Choke? I say Choke. Is their a downside to the Choke?

I guess I should take this somewhere else stead of high jacking the thread.
Where should I take this line of questions for new setup Management?

 

[brandon7861]

Feel free to keep it here, this thread was going nowhere anyhow. I just wanted to know why a select few had issues with LMR400, but the reality is all those people could have different problems.

A choke will not hurt anything and is cheap insurance against common-mode currents. You could always wait to see if you need it before adding it, but there is no harm in having it there from the start.

 

[unit_399]

I think that the "problems" with coax are overblown. Most any full size 50 ohm coax ( RG8, RG213, LMR400, etc.) from a reputable manufacturer will perform well if installed with properly soldered connectors, sealed against water ingress. When setting up a station (base or mobile) if you cut corners, you can expect problems. For best performance make EVERYTHING 100%+ , and you will be good to go. My credo is, "If you are not certain something will help, but you know it won't hurt . . . DO IT !!" A bunch of small improvements add up to a big improvement. It's what makes the difference between a good station and a great one. The object is to make sure that you get every last milliwatt out on the air.

- 399 J.J.

 

[edfiero]

I used with a maco 5/8w. no problems, other than it was stiff as a witch in January and almost as bad even in August.