Tuning 1/4 wave on Bike

[C2]

I've had this SS stinger for the broadcast band laying around with plans to cut it for 2m.

My bike has a sissy bar, a passenger seat back, so picture an upside down U bolted just behind the seat. The top of that is where I bolted an angle bracket for the stinger.

My first measurement is taken with a sweep from 100 MHz to 200 MHz, and I see a VSWR valley right about 100 MHz, exactly where I'd expect for a 31" long stinger, and a bump around 150 MHz. I set the SWR meter for the 2m band and SWR is high, around 12.

I cut off 10 inches and SWR came down to about 9. I cut another 1/2 inch, not much change. I cut another inch, not much change. I reset the SWR meter for 100 MHz to 200 MHz, and my valley is still at 100 MHz with the hump still up around 150 MHz.

WTF? How do I tune this POS? How can it be the same after reducing the length more than 1/3? Now I've likely cut too much off!

I'm grumpy now, didn't even ride the bike to work today.

 

[binrat]

I always use this http://www.csgnetwork.com/antennaevcalc.html as a starting point then I go from there.

 

[Robb]

1/4 wave @ 2m = ~19 1/2"

??

 

[C2]

1/4 wave @ 2m = ~19 1/2"

??

That's right. The stock antenna was 31" and I cut off about 10" trying to be a little conservative (~20").

So, with a 31" stinger, my SWR comes in at ~ 105 MHz.

with a 19" stinger, my SWR comes in at ~108 MHz.

WTF is that about?

Looks like I need to cut off about 20 more inches, eh?

 

[C2]

I would have hooked it up and rode it in, but the SWR was still around 9 and I did not want to blow out the PA on my little APRS xceiver...

Oh, and I have about a 1 foot jumper with a fresh BNC that I just put on, mini 8X cable soldered to a couple lugs at the antenna side.

 

[binrat]

How big is the "U" shape bar??

 

[C2]

How big is the "U" shape bar??

It's an average sissy bar, comes up to about the shoulder blades (perhaps 20 inches tall). The sissy bar ends bolt to the rear fender.
Looks like this.

 

[Robb]

Not much to a 1/4 wave antenna - regardless of what band it is cut for. The ground plane angle will change the feedpoint impedance. Perhaps adding a couple of ground planes to it and re-check the tune. Can't say that a hairpin shaped sissy bar is not going to work; but at this point one has to reconsider each factor.

Soooo, having said that, I think I would need to recheck all of the connections and harness and find a problem there. No where else it could be . . .

 

[Moleculo]

I'd bet that there is some type of coating on the chrome that is preventing from getting a good electrical ground. I had a similar problem with the rear ladder on my RV and ended up running a ground strap to the frame.

 

[RatsoW8]

I think Robb was closest on this. You'll need more of a ground plane for the 1/4 wave antenna to work correctly. Can you post a close up picture of the mount?

I was going to install a 2m rig on my Harley and use a ground independent vhf antenna of some type. I just never got past the planning stages.

 

[C2]

Now, I'd be ecstatic if it was just a grounding issue, and I could see it being the problem. Actually, the back of the sissy bar has 3 flanges that bolt the cushion, and I'm using the one on the center top to hold my aluminum bracket, and flanges are chrome plated and a bit slippery. I'll hang a pigtail off there and see how it reacts.

 

[C2]

I tried a few ground wires, but measured continuity to the engine, so grounding should be OK.

I kept trimming and watched the two nulls, one around 100 MHz and another around 250 MHz. The nulls moved only a couple MHz if at all, but the SWR started to rise on both nulls as I trimmed. The whip is now 16" so far too short to be useful now.

I took the antenna off and bolted it to my steel shelving unit (it is 15 feet long and 5 feet tall). The SWR curve was exactly the same as when mounted on the bike.

I pounded a sleeve onto the end and tapped a 3" tip back on. It makes no difference if I remove the 3" tip other than just make the SWR curve go UP! The nulls stay in the same place.

 

[Robb]

Add two or three 19 1/2" radials placed 180 degrees or 120 degrees apart and bent down at a 45 degree angle. You can use use wire coat hangers for materials in order to test. That should work unless there is something else wrong with your arrangement . . .

 

[C2]

The antenna is bolted to a steel frame shelving, so there are lots of "radials". Well, at least three, but of course the cross members are just set in slotted holes.

I brought the thing in to work today to use the expensive equipment. We just need to find something to bolt it to for a ground plane. I'm not sure why I'm bothering with that, probably because the metal shelving produced the same SWR response as on the bike.

I'm not sticking ground plane radials on my motorcycle. It either works simply or I forget the whole idea. I might be better off just buying a decent 2m handie antenna and just toss the whole APRS module in the drawer, because it ain't gonna work around here without more power anyway. I'll just install firepin on my cell phone and get what I'm looking for without all the hassle.

I did manage to get an image of something, totally crappy image. I'm kind of useless today, well, yesterday too...

 

[C2]

We used a real antenna analyzer and hooked up the coax with the antenna sitting on the bench, and it came in nice around 160 MHz.

We walked around to various metal things and stuck the bracket against it while measuring. A couple of first class dorks! I felt like a boob fiddling around with tin foil on rabbit ears trying to get the snow off the screen (cause the audio always came in fine).

I have no idea the purpose of what I'm doing here. I learned nothing and accomplished nothing other than ruining a nice SS whip.