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.

 

[FlatlandBusa]

I run 135' of Davis bury-flex 400, that is direct buried in my lawn. The ends are waterproofed per commercial standards and it has been in service for 2+ years now with zero issues. Currently running 1.2KW PEP through it, to an IMAX2000 that I use on 10, 12, 15, & 17 meters, on 17 meters the SWR is above 3:1 before the tuner.

An update to this, currently running legal limit+++ to this setup with zero issues.

 

[BC Coyote]

I have a 120 foot length of Belden 9913 RG-8 cable here that is well over 25 years old and still in daily use feeding my Antron 99. No problem with it whatsoever; it's just as good as the day it was made.
I also have some other coax here that I have on a scanner antenna that is nearly 40 years old ! It is Tandy RG-8 that I bought new from Radio Shack in 1987 for a base setup I had at the time, and though it has faded to grey and has stiffened over time it still works fine for a receive setup.

So long as coax cable doesn't get kinked badly and doesn't get water damage it should theoretically last a very very long time....... and that goes for all but the cheapest coaxes.......

 

[W4KVW]

I run LMR400 on all bands from HF-440 mhz with Zero Issues. I use the ultra flex even on vertical antennas because I want the flexibility it provides with the stranded center. You can't be too careful. Many forget that you don't want non Ultra Flex on any antenna system that is being rotated. LMR400 is Not the Best that you can buy but it's more than most will ever need. I run 100 to1200 watts thru several runs on HF & 6 meters so I'm not cutting it any slack & it does not let me down.

 

[Rwb]

Hard to believe, I fried Imax with 800W PEP

i must agree

 

[Rwb]

i got lots of lm 400 rolled up and next load of scrap,its going to the recycle center.
i had both a 99 and gp 9 fed with that stiff pima coax. swr fine resont. but seemed for what i had was lacking. i found some old school fine center wire coax and very fine braid. changed coax to a 99 n several said i gained 3 s units. then the gp 9 got it too and i gained even more . everybody i talk to said i really came up like 5 to 6 s units.
im thinking n probably wrong but dont current flow on outside of wire? humm hardline single wire,never mind my ramblings

 

[Rwb]

im thinkin more about current flow on wire. dont electricity and radio signal flow on outside of wire? if so several smaller wires would definatly have lots less loss as there is lot more surface on several thin wires in same diameter as 1 hardline wire?

 

[Captain Kilowatt]

im thinkin more about current flow on wire. dont electricity and radio signal flow on outside of wire? if so several smaller wires would definatly have lots less loss as there is lot more surface on several thin wires in same diameter as 1 hardline wire?

On any reasonable run of cable the dielectric losses of the cable will far outweigh any ohmic losses in the centre conductor. IOW.......don't worry about splitting hairs over whether the centre conductor is solid or stranded unless a rotator is involved and then there is NO question that stranded is better simply from a mechanical aspect and not electrical.

 

[Greg T]

Personally, I don't get the hype behind running 1/2" coax unless you're running 3kw or a 400' run. I've used LMR-240 for many years and have used 1500 pep through it without any issues. It only has 1db loss at 100' @ 27mhz and it's less than 1/4" diameter. I'm still using 240 today and see NO reason to switch.

https://www.pasternack.com/images/ProductPDF/LMR-240.pdf

 

[fourstringburn]

I looked at it this way,

I spent good money on a HF rig, HF amp, and a multiband beam antenna, why go cheap with coax?

I use 1/2 inch hard line with N connectors.

Maybe many of you should consider the same with your base antenna?