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coax length-LDF4 heliax

G

Gearhead

Member
Feb 4, 2007
76
0
16
Does coax length matter on a base like it does in a mobile (for example 3 ft. increments)?
My antenna is I-10k roughly 58' from ground to bottom of antenna. Coax is Andrew LDF4 and will go down the length of the tower, almost to ground, where the shield will be grounded, then go back up the house to the 2nd story, for an approximate total length of 75ft +/-. IS there a magic number for length, when it comes to this kind of coax? Jay's instructions says he uses 50 ft. of LMR400 to tune the antenna (which I also have), but will the tuning change, when I switch to the LDF4 and the longer length?
Here's some specs on the heliax:

Cable Impedance 50 ohm ± 1 ohm
Capacitance 23 pF/ft | 7579 pF/m
dc Resistance, Inner Conductor 1.480 ohms/kft
dc Resistance, Outer Conductor 0.580 ohms/kft
dc Test Voltage 4000 V
Inductance 0.058 µH/ft | 0.190 µH/m
Insulation Resistance 100000 MOhm
Jacket Spark Test Voltage (rms) 8000 V
Operating Frequency Band 1 – 8800 MHz
Peak Power 40.0 kW
Pulse Reflection 0.5%
Velocity 88%
Frequency (MHz) Attenuation (dB/100 ft)
20 MHz 0.291
30 MHz 0.357
Thanks for any help!
 

If the feedpoint impedance is close to 50 ohms (measured right at the feedpoint directly; not fifty feet away through coax), the feedline length won't matter at all, as long as it's the same impedance as the feedpoint.

So the antenna is maybe 54 ohms (and very lightly reactive) and the coax is actually 49 ohms? No problem. Get on the air and be happy.
 
Coax length does NOT matter whether base or mobile if,and I say IF, the antenna impedance is 50 ohms or very close to it.There are the odd times when a mobile antenna will not tune and the SWR at the radio can be changed by changing the cable length but in all cases a base antenna should be able to tune to 50 ohms.The best length of cable is whatever it takes to do the job. ;)
BTW nice cable. Overkill but nice.I suppose I shouldn't talk.I run LDF4-50 on 2m,6m,and my HF beam. :oops:
 
Gearhead said:
Does coax length matter on a base like it does in a mobile (for example 3 ft. increments)?
My antenna is I-10k roughly 58' from ground to bottom of antenna. Coax is Andrew LDF4 and will go down the length of the tower, almost to ground, where the shield will be grounded, then go back up the house to the 2nd story, for an approximate total length of 75ft +/-. IS there a magic number for length, when it comes to this kind of coax? Jay's instructions says he uses 50 ft. of LMR400 to tune the antenna (which I also have), but will the tuning change, when I switch to the LDF4 and the longer length?
Click to expand...

Gearhead, what has been stated here is correct, so the line should not make much difference in the case with the I-10K.

In my very first tuning efforts with my I-10K I think I saw something similar to what Bob85 has stated to us in another thread here on the forum. He is still planning to get back to us on that effort however. I made every effort to mearsure everything exactly as Jay indicated in his well written docs. This is not to say that the antenna did not work well, it did, but maybe the differences I saw with my analyzer were due to something affecting the antenna and that I could not see.

I tested at 13' in height and I was not far off form nearly perfect results from my Autek analyzer, but my I-10K was high in frequency at R=50 ohms @ 27.500 on resistance and low in frequency X=0 ohms @ 26.970 on reactance using my Autek reading at the feedpoint. Regardless, I doubt this made one bit of difference to the effective operations of the antenna at any height.

Using a 50' working line and my inline meter produced an SWR very slightly curved to a bit high in the CB band. For me it was pretty easy to adjust TI the resonance down, but was more difficult in affecting the resistive match. I think this has more to do with the physical procedures necessary to alter the T2 tuner and not affecting T1 a bit at the same time. Maybe when Bob85 completes his testing he will be more clear on this.

If you tune using an inline meter and the 50' line Jay suggests and you are careful to get the measurements Jay suggest to start, you should be fine and the added line will probably make things look even a bit better than reading at the feedpoint.

We are just lucky we don't have to be perfect and there should not be any noticable differences even if you are off a bit more or less from perfect.

Mine does not allow me to hear DX when there is none like KingCobra does coast to coast, but it is a great antenna and you will be happy with it.
 
Thanks guys. I know in theory, that it should'nt matter, but I've always had problems with my mobiles and even my current base setup. I also was'nt sure if heliax would change anything because of attenuation, velocity, etc.
I'll just follow Jay's measurements, and run just enough cable to put my antenna switch in a convenient location.
BTW, I chose to run LDF4, because I already had it, lol. plus, I already have the connectors for it. I have LDF5,6,7, FSJ4, LMR400 LMR400 superflex, and can get CommScope and eliptical waveguide also.
 
Well Gearhead, you let us know what happens when you change if you do, OK?
 
Marconi said:
I made every effort to measure everything exactly as Jay indicated in his well written docs.
Click to expand...
I use Jay's math to set the main radiator and the ground radials. I don't measure the trombones however. I use the analyzer and adjust until its where I want it to be.

I wouldn't get hung up on the trombone measurements as there are too many variables that could affect their physical length. Jay's numbers get you close, and I believe he mentions that in his instructions.
 
I can't put the tower up, until next weekend :( , but when I do, I want it to be right. I never realized how much it costs to have a decent antenna and tower system!
I-10k =320
used tower=150
concrete =80
phillystran kevlar guy wire =240
guy anchors =195
guy bracket =135
turnbuckles =126
2 LDF4 UHF connectors =80
LDF4 coax =free
3 8'x5/8 ground rods =free
#2 solid ground wire =free
= $1326

That's even with all the free stuff, and I still need a mast pipe :shock:
I did overkill some of it, but what good is an antenna that can handle 100 mph winds, if the tower can't?
 
your I-10K exellent antenna

your LDF4-50A exellent coax

regardless if talking home install or car install
the needed length in coax that best suits ones needs

is Always the shortest length needed

well at least i wasn't one to bring that up again people

:peace :peace
 

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