Coax at the bottom of a Tram 1486.....

[Gnarlykaw]

I am just a couple days away from putting up a base antenna.   Its going to be 34ft off the ground, on a flagpole...

Flagpole has a grounding clamp, AND No.8ga wire going to a grounding rod.  The coax is LMR400, and it will have a lightning arrestor coupler in line with a 8Ga. lead on it going to the grounding rod.  The question is, do I need to place the loop in the cable at the Antenna?  I was under the assumption that is a drip loop?  the connector is shielded in a collar/ base mount for the antenna.  The radio will be attempting to squirt somewhere between 38-44 watts....

[MarkInTampa]

Both my Comet GP-6 and GP-9 manuals show a loop at the base of the antenna. I don't know why but I put them in because the instructions showed to do so.

 

[WRYZ926]

1 hour ago, MarkInTampa said:

Both my Comet GP-6 and GP-9 manuals show a loop at the base of the antenna. I don't know why but I put them in because the instructions showed to do so.

 

The purpose for the loop right below the antenna is incase you ever need to remove the antenna from the mounting bracket. The loop gives enough slack to pull the antenna out of the mount and disconnect the coax.

And you definitely want a drip loop in the coax where it enters your home to keep water from running down the cable and then inside.

[gortex2]

In my 40 years in the commercial LMR world I've never installed an antenna with a loop below it. Not really sure why folks think its needed. None of mine at home or at any of my tower sites have a loop. Most have a drip loop near the bulding enterence but many have proper antenna line feedthrus to eliminate water ingress at the location. 

[warthog74]

I use a loop on all my base/repeater antennas. The main reason besides routing water away is that the loop also acts as a strain relief that prevents the weight of the coax from constantly pulling on the connection at the base of the antenna. Good coax is quite heavy and will eventually slip out of zip ties, tape, etc from wind, water, age, etc. If your coax is thick and not easily bendable (like LMR-400) make a "C" shape. I look at it as "better safe than sorry". It certainly doesn't harm anything. Below are images of my base antenna and my neighborhood repeater antenna.

[gortex2]

If using long chunks of feedline they should be held in place with hoisting grips. That hold the cable to the mast or tower.

https://www.talleycom.com/product/ACYHG-PL-12

[WRYZ926]

As stated, there are a few reasons why one would want to put a loop in the coax right below an antenna. Though it is not necessary if the coax is attached to the mast/tower so that there is no strain on the connectors.

I personally put a loop in the coax on all my vertical antennas just so I don't have to undo any of the cable clamps/supports if I don't have to. 

Yes a few manufacturers to include Comet, Diamond, and Tram recommend a loop. 

[rccooper4]

I usually leave some slack at the top weather it is a loop or not. It will give some extra cable just incase you need to change the connector up there. It is better to have some slack to put a new connector on than replace the coax because its too short. I am currently using a antenna with the so239 connector on it but thinking of replacing it with one that comes with the N connector, so I would have the cable up there to change my coax to a  N connector. 

[Gnarlykaw]

ive seen several with a loop at the bottom...

 

[WRKC935]

I will say this about any sort of coil of cable right under the antenna or anywhere else.  It is going to be an inductor to lightning.  If you properly ground the antenna to the mast or with a ground wire for non-conductive masts, then it probably doesn't matter.  If not, then it may draw lightning more so than an antenna that has no inductor directly below it.

I believe the reasoning for it was it's a 'service loop' if your connector goes bad or you change antenna's.  Gortex mentioned they don't do this on commercial installs.  And that's due to there typically being two cables in the run.  The larger vertical run of cable up the tower and then a smaller, more flexible jumper  from the top of that run out to the antenna.  At least that's what we do if the feed line up the tower is 7/8 or larger. 

Hams probably started doing it because of the experimentation involved with ham radio and cutting a cable to length limits what can be put up in place of the old antenna, and jumpers and the like cost money they might not be willing to spend. 

I have seen 7/8 out to antenna's, but they are antenna's designed with a pigtail cable to feed them.  It doesn't work well for antenna's with fixed connectors in the base of the antenna, and depending on the cable size and antenna design, the connector might even be to large to enable the connector to be threaded up to the antenna, like the Station Master antenna and a run of 1 1/4 or larger.  THe connector for that antenna is recessed into the bottom of that antenna and a 1 1/4 connector would not directly connect to that recessed connector.  There is just no room to get a wrench in there and tighten it.

 

 

[SteveShannon]

 

33 minutes ago, WRKC935 said:

I will say this about any sort of coil of cable right under the antenna or anywhere else.  It is going to be an inductor to lightning.  If you properly ground the antenna to the mast or with a ground wire for non-conductive masts, then it probably doesn't matter.  If not, then it may draw lightning more so than an antenna that has no inductor directly below it.

 

Can you please explain how that works?  I agree that a single large coil will have an inductance, but how does that draw lightning?

[LeoG]

Reasoning that the weight of the cable can pull on the antenna or fitting and damaging either could have some merit.  But as a drip loop that seems silly.  Of course it will work as a drip loop.  But the whole assembly is in the rain and will get wet.  There is no reason to have the drip loop any place other than where the coax enters a building to keep water from penetrating.

As for the loop acting as an inductor and attracting lightning I find that a bit hard to fathom.  One loop of shielded wire isn't much of an inductor and the whole of the height of the antenna in the air is going to be the main attracter of lightning, in my opinion.

Grounding the antenna will attract lighting to as it puts the ground potential up in the air.  Again inconsequential because we are talking miles of distance and maybe 30-50' of rounded pole rising into the air.

 

[WRKC935]

15 hours ago, SteveShannon said:

 

Can you please explain how that works?  I agree that a single large coil will have an inductance, but how does that draw lightning?

I guess 'draw' isn't a good description, other than an inductor looks like an open (not ground) the instant power is applied.  The bigger issue is if it takes a hit right at the antenna, you get hit by the lightning, and the inductive kick.  So you get blasted twice. 

Mind you that it seems the ham mentality is if a single service loop is good, then rolling 15 feet of cable up is better.  And if a pre-made cable is used, there is just no way it can be cut to length.  So it gets coiled somewhere.  I have seen more than once where someone bought a 100 foot pre-made cable to go 30 feet and teh other 70 feet is in a coil someplace.  Sometimes its at the base of the tower, sometimes at the entry to the building, and ever behind the operator desk.  As a commercial radio guy, this drives me nuts.

 

[Socalgmrs]

I don’t  personally know of any antenna that has a loop.  Not a one.   
 

sorry to hear about the tram btw.  

[gortex2]

Apparently the loop idea is yet another ham based idea dragged into GMRS....