Ham antenna question

[jonheld]

I have a decent setup in my 80 Series. I use a Diamond K400CNMO mount and 2 different Diamond antennae NR72BNMO and NR770HBNMO depending if I'm in the trees or not. They both work extremely well and I can get really good performance out of my shorty.

A few weeks ago I purchased a Larson 150 NMO 1/2 wave whip, and the performance seems to really suck as compared to even my shorty Diamond. I have trouble hitting repeaters that are easy to hit. I was expecting the exact opposite since this is a very tall 1/2 wave.

Is there a difference in NMO mounts? I assumed it was a standard.

The Larson seems to fit properly and securely. There is very little adjustment possible at the mount where the whip fits into the base. The 2 set screws are nice and tight and according to the piece of paper that came with the antenna, no trimming is required for 2M.

 

[george_tlc]

An antenna analyser (for bigger $$) or a decent SWR meter will help you decide whether the antenna is at least tuned correctly.

I'm NOT a fan of those 2 little grub screws that hold the whip into the NMO base. I have a shorty Larsen and on a trip into the oz bush with repeated tree/branch strikes the whip worked itself loose (grub screws were still in place) and was lost.

Props to Larsen (part of Pulse), I sent an email and after a few back & forths they sent me a brand new replacement (the whole unit, since the whip isn't available on its own). I then marked the position of the whip, remove it, used my dremel to make a slot that lines up with one of the grub screws, reassembled with some loctite (which the nice lady that responded to my email recommended... of course they don't use it at the factory....). to put it together. At least the grub screw now sits in a slot to hold the whip securely.

The other issue is the 2 grub screws really do NOT work together as a backup. If you tighten #1, then tighten #2, then loosen #2, the whip will come out. Since there is some 'slop' in the hole in the base, tightening #2 moves the whip a little and #1 is now no longer tight. So, basically only 1 grub screw is really holding it in worst case.

I went with the Larsen since it was recommended as being very rugged for my use (low hanging branches etc) and it was dual band. On my US rig I have always used Comet and been happy.

The Larsen is 'preset' at the factory and that's done with those 2 grub screws, since the whip can be moved quite a bit up/down in the base. It would have been VERY NICE if Larsen put a paint mark etc to show the position of the adjusted whip so at least there's no doubt by the user whether the whip may have moved over time.

Anyhow, no real answer for you, but more just stating some possible issues with the whole 'factory preset tuning' and the use of those grub screws....

cheers,
george.

 

[jonheld]

Yea, I know I should get an antenna analyzer, but the 2 Diamond units were great right out of the box. I went with the Larson because of good reviews here and other places. I wanted a good durable antenna that could take a beating in the trees and come out OK. The Diamonds bend at the curly Q.

I have to get Ramy to send me his analyzer and play with it.

 

[jonheld]

Went through the length chart again that came with the antenna. It's 49" from the factory which is for 144 MHz. Since most of my stuff lives in the 146-147 range, I cut an inch off. Seems to work better now. Hard to say without an analyzer.

 

[sdnative]

Are you using the same mount as the known good antennas were using (i.e. have you ruled out the possibility of the issue with something other than the antenna)?

I think the Larsens have a little spring loaded pin that makes contact with the center disk of the mount. Make sure this is working properly and is applying good force to the mount. I think the Diamonds have a tab instead that needs to be bent out slightly from time to time.

I had a Larsen 27mhz CB antenna mounted to a bumper NMO mount and it had horrible performance. Turns out the mount wasn't grounded to the bumper very well. I then borrowed a meter and measured >20 SWR!

 

[jonheld]

Yes, the only thing that changes is the antennae. The Diamonds have no such tab. At least not the 2 that I have. They have a spring loaded center pin.
The Larson has a tab which is sort of silly to me, but I guess you get what you pay for.
I may try Comet next.

 

[fj4068]

Is this the 1/2 or the 5/8 model? This thread says 1/2 but your thread on the analyzer says 5/8. If it is the 5/8 model, I wonder if the lack of a good counterpoise at the hatch mount location is causing the disappointing performance?

 

[jonheld]

I think the problem here is a loose nut behind the keyboard

I remeasured and looked again at the base coil for the Larson. It is a 1/2 wave that I cut to 5/8 specs. Apparently I wasn't really paying attention to what I was reading or what I was measuring.
The first cut was to 48". This would have been correct for 5/8 at 146 MHZ. Last night after looking at my invoice and comparing it to the base coil, I realized my mistake. I cut it down to 40 1/8". I will have to test it to see what kind of difference it made.

Sorry for the misinformation on this thread.

 

[sdnative]

Glad you got it sorted out. We've all been there.