Antenna Springs

[Lifestyle Overland]

Hey folks, looking for some advice on the proper way to add a spring to the base of my dual band (2M/70CM) Larsen antenna.

Here's the part numbers I'm using:

K515SNMO Diamond Mount
NMO-2/70B Larsen Antenna
SPRINGB Larsen Spring

My question is this: do I need to cut the antenna so that the overall length matches the original length of the antenna? The interior of the spring has a braid so the spring itself isn't necessarily part of the antenna.

 

[vicali]

I would install it then tune it. google pic;

I've got the same 2/70B antenna myself on the fender;


I don't get into the really thick stuff but I've never worried about the odd branch - I've also seen these Larsons on forestry bush trucks go for a long time with not one problem at all. Are you sure you need the spring?

 

[GFB_Steve]

My question is this: do I need to cut the antenna so that the overall length matches the original length of the antenna? The interior of the spring has a braid so the spring itself isn't necessarily part of the antenna.

Trimming the antenna to keep the overall length the same would be ideal. It would help to use an SWR meter to tune it, but if you're already getting the range you need, you'll probably be fine after adding the spring and trimming.

 

[Lifestyle Overland]

I don't get into the really thick stuff but I've never worried about the odd branch - I've also seen these Larsons on forestry bush trucks go for a long time with not one problem at all. Are you sure you need the spring?

I've been in stuff bad enough to lay them flat so the spring really needs to go on at some point.

Trimming the antenna to keep the overall length the same would be ideal. It would help to use an SWR meter to tune it, but if you're already getting the range you need, you'll probably be fine after adding the spring and trimming.

I took the spring off for my trip to the Mojave Road and found that I get much better reception without it. I still need to put an SWR meter on it and see what I'm working with.

Here's another question, should I run a ground strap from the mount (which has a rubber clamp) to the roof rack? I get so much conflicting info on the ham forums!

 

[vicali]

I've been in stuff bad enough to lay them flat so the spring really needs to go on at some point.

Why not carry a ducky for when the trails get that bad?

or even an NMO cap if you can shut off your radio;

Here's another question, should I run a ground strap from the mount (which has a rubber clamp) to the roof rack? I get so much conflicting info on the ham forums!

Roof racks are pretty notorious for being lousy mounts, both electrical ground, and radio ground plane wise.

 

[GFB_Steve]

I've been in stuff bad enough to lay them flat so the spring really needs to go on at some point.

Maybe your solution is 2 antenna mounts? One up high with little to no obstruction and another down low where the antenna won't get broken off by branches and such. When you're in a wooded area, run the antenna on the low mount, when you're in open areas, use the high mount. As long as you can get to the back of the radio, swapping from one to the other only takes a couple minutes.

I took the spring off for my trip to the Mojave Road and found that I get much better reception without it. I still need to put an SWR meter on it and see what I'm working with.

find a cut-chart for your antenna. It will tell you what length to cut your antenna to for whatever frequency you use. Also check the cable with a multi-meter. There should be zero continuity between the center conductor and the outer shell (shield).

Here's another question, should I run a ground strap from the mount (which has a rubber clamp) to the roof rack? I get so much conflicting info on the ham forums!

Grounded is better but not necessary. More importantly, are you using a non-ground plane antenna?