Ham antenna questions

[Mike Weiss]

Hi all. I am in the process of updating, upgrading my antennas(CB, ham, scanner). My questions pertain to the ham side of things.
Currently I have my ham antenna mounted using a 3/8 x 24 type mount on a behind the cab headache rack. Through discussions on here, I want to upgrade to a NMO mount, preferably with a "L" bracket behind the cab of the truck. I am looking at a Diamond 7900 or a comet SBB7. Both are 55 to 60 inches which would provide adequate clearance above the roof line. Or would it be better to mount the NMO through the roof, and use a shorter antenna. I am worried about ground plane, etc. and I dont know about the difference in 1/4, 1/2, and 5/8 wave.

thanks in advance for any help.

Mike Weiss/KE5NXJ

 

[David Drummond]

The SSB7 is an awesome antenna. That said, I wouldn't mount it on the top of a truck. It's a relatively stiff antenna and I nearly broke it a number of times before I took it off and went back to a whip.

For the NMO you would prob get a better ground plane in the center of the cab roof, but harder to put in the NMO of course and having to modify the body to do it. I'm not sure if in this case the payoff would be enough change in your swr as opposed to just using an nmo bracket on the rack.

 

[D. Hayes]

I'm no expert but from what I've read 1/2 wave will be better suited for poorer ground plane situations where 5/8 is better to match to coax. So I would go with the 1/2 wave if it wasn't mounting centered in ground plane.

Far as the NMO I can't see how that is an ungrade but just a different mounting option. The real question is, can you even use those larger antennas with a NMO mount? Just having the antenna above the roof line would yield better results than blocking the antenna by having it below the roof. I'm using a off center mount and I have my antenna (which is multi-wave) attached to the side of my roof rack and it seems to work really well.

You can in fact get folding mounts and even electrical folding mounts that lay down at the push of a button.

 

[Mike Weiss]

the SBB7, I belive, is a fold over antenna. I wanted to put on top of truck. But I can get better gain, etc with the longer SBB7, but the draw back is the height. Thats why I was looking at mounting it on top of bed where the top will clear the roof. Hope that makes sense. I was just worried that it doesnt have a ground plane as it would if I mounted it on top.

 

[David Drummond]

Indeed it is a foldover. But to do so you have to get out and unscrew a little thing, pull up and fold over if I remember correctly. It was a pain in the ass basically. I whacked it on trees and everything else. I'm surprised I never actually broke it.

Dory also made a good point. That antenna is a big cumbersome to mount just on roof sheet metal. It was flexing my roof pretty good. Mounting on bed behind cab would probably give you some forward transmitting issues, but would negate that tree whacking issue. The bed of the truck behind it would probably give sufficient ground plane. I think you would attenuate a lot of your forward signal being behind the cab though.

 

[John Wetter]

The SBB-7 is indeed a great antenna. I've had mine for 4 years now and it works better than any other antenna I've had. As with all ham antennas, the best height you can give it and the best ground plane possible will yield best results. Having it mounted below the top of the cab will get a lot of RF into the truck and you'd also want to test the SWR on it with the different mount. It is designed to use a ground plane. It is a fold over, and I use that. You just have to lift it up (it's snug and spring loaded) and fold it over. Depending on the mount, they also have mounts with tilts on them for getting in the garage, etc. I've whacked mine on the garage door many times and it's still working like a champ!

The difference in wavelength on the antenna gives you an idea of its gain. A better idea for this would also be to look at the numbers of how many dB of gain it will give you. The more the better.

 

[Tim Stoecklein]

I'm not familiar with the specific antenna you're talking about, but some great points have been mentioned. While a large antenna should increase your gain, it sounds like your mounting options could yield the same results by mounting a smaller antenna in the center of your roof compared to the larger antenna bed-mounted....but I'm no antenna guru.

David brought up my biggest concern, you'd likely need to reinforce an NMO mount for the larger antenna. I'd go with a smaller antenna center-mounted if I was you.....but I'm not I do, however, get about the same practical performance from a 1/4 wave mag-mount in the center of the roof that I get from my 5/8 wave lip-mounted on the back hatch of my SUV.

Good luck!

 

[Rob Wadsworth]

Since we are talking about 2 meter, the size of the ground plane isn't such a critical factor. For a CB - yes. I know people who put a mag-mount 2 meter antenna on top of a refrigerator and receive and transmit very well. Sure; more surface area is better. I think that if you have an issue with overhead clearance; then you might need to have it fold it down.

I would personally go out of my way to avoid cutting on sheet metal if I could.
I would also go - without hesitation - to the antenna that has the best receive and transmit specs.
Compromising efficacy for durability makes for tough choices.
I would go for better TX/RX quality every time...

 

[Mike Weiss]

Ok...I've got some good ideas on what I need to do. BUT, I have one more question...What would be a good antenna (2M or dual 2/70) that does not require a ground plane that I could mount on my headache rack. any ideas on that?

 

[Tim Stoecklein]

Mike, this is the one I have attached with a lip-mount to my SUV:

http://www.rfparts.com/diamond/nr770hbnmo.html

It's an NMO with the ability to fold over. I've had great luck with this antenna without a ground plane.

One other option you might consider if you're attached to a headache rack, you could still make a small ground plane plate to attach to the rack......just a thought.

 

[Mike Weiss]

Tim..thats is one of the antennas I have been looking at today. It looks like the one I might go with, but you have my curiosity peaked. My headache rack is a louvered style behind the cab and I had a 5" plate welded to the top for lights, instruments, etc. I have been using standard cb mounts(3/8x24). My ham ant. is on the right with the other ant for my scanner and cb on the far left. the scanner is offset on the left 8" from the cb. I was told that with my mount and what I was using, it should work decently as long as I have a good separation from the other ant. So how would I make a ground plate, or would I even need one, if I kept this type setup.

 

[Rob Wadsworth]

I use the NR770 with a mag mount - which uses capacitve coupling to the sheet metal for a ground plane. One of the better Diamond antennas to use. Probably work great for what you have in mind.

I seen the Sean's/TIV's "DOGHOUSE" team using a Diamond X-50 2m/440cm base antenna on their spotter vehicle. That is what I use for my 2m base station! Bet they don't go to the drive-thru for a burger...lol

 

[Tim Stoecklein]

Mike, it sounds like your 5" addition to the typical headache rack frame would suffice if you want to use a mag-mount. That being said, don't skimp on the magnet, and make sure it holds solid if you go that route. Not much fun to have your antenna bouncing of the side of your vehicle, or busting out a side window, if it can't hold true.

Your other option with this particular antenna is to bolt an L-bracket NMO style mount directly to the rack. This style would be SOLID.

Someone can correct me if my belief is incorrect, but I believe the NR770 is what they call a "ground-independent" antenna, meaning it can perform as well with a lip-style or L-bracket mount independent of the ground plane we typically associate with a center-roof mounted antenna. I've had this antenna for a LONG time, but I think this aspect was a selling point when I was researching antennas...I just can't remember that far back

 

[Jason Foster]

I seen the Sean's/TIV's "DOGHOUSE" team using a Diamond X-50 2m/440cm base antenna on their spotter vehicle. That is what I use for my 2m base station! Bet they don't go to the drive-thru for a burger...lol

I also believe TWISTEX used X-50 or similar fiberglass high gain antennas on their vehicles. These have small radials and do NOT require a ground plane. However, they tend to be large and sensitive to position (vehicle sheet metal if too much adjacent to the vertical 'whip of antenna). Also these do very poorly with lightning, tend to explode whereas other metal shaft antennas don't suffer as much damage during a strike.

I run a Diamond SG7900 on a PL. I prefer the PL because it's threaded shaft is longer and I believe performs better in high wind conditions. Newer versions are tilt-over also and I believe do not require a ground plane, but verify that last factor, I'm not sure (mine has a ground plane)

 

[Joe Dorn]

Ok...I've got some good ideas on what I need to do. BUT, I have one more question...What would be a good antenna (2M or dual 2/70) that does not require a ground plane that I could mount on my headache rack. any ideas on that?

See the previous thread for an extended description of my set up.

I use the Diamond motor driven mounts and their cables and antennas for my two meter voice communications. These do not require a ground plane. The mounts can attach to a luggage rack or door lip. (you have to look close in the catalogs to find the Diamond motor mounts and cables.)

I also will not buy a dual bander with the 'Bakelite' antenna coils in the middle. I have busted more than one at $30.00+ a pop. My dual branders have the coil in the antenna proper (the antenna wire itself is coiled in the middle).

I am not as concerned about my voice communication as I am about my APRS communications. The repeater antenna is usually the controlling factor on range. The APRS digis on the other hand are frequently mounted at best on top of a tall roof. I use a 5/8's wave mag mount in the middle of my pickup roof for APRS.

I have used the hole in roof mounts previously and except for neatness of the installation see no advantage. I also used the NMO mounts previously but had moisture and contact problems with them. I now prefer the Pl-251 type mounts wrapped in Co-Ax seal.

The mag-mount I use with the 5/8's antenna takes a crowbar to make it let go of the roof.

I have an MFJ antenna analyzer and check all of my antennas for SWR at installation and at least once a year thereafter.