Ham: Test of SWR / various antenna locations on Cruiser for 2m

[e9999]

well, spent a bit of time measuring the SWR on 2m, by moving the antennae on mag mount all over the 80, both a 1/2 wave and a 1/4 wv.

I saw wide variations in SWR, some very bad depending on location. Note also the large variations in SWR between 144, 146, and 148 MHz.

Have to say that the situation is a bit special cuz I have a full size rack with mesh floor on, so it's a bit more complicated than just plopping the antenna in the middle of the metal roof.

Best locations for 1/2w was on a metal bridge over the rack, on middle of rack, on hood, and on ARB bull bar, all about same SWR. (Hood and ARB not so great as it will transmit right in your face and is lower.) For the 1/4w it was directly on the roof in the middle, under the rack.

I'll try to put up a screenshot of a spreadsheet I did on this

unfortunately the SWR meter I have does not do 70cm.

This was done with a mag mount, so with some capacitance coupling to the metal / ground plane. Not sure how different the numbers would be with a regular mount. But FWIW, I tried the rear edge of the roof, so perhaps an indication at least on a relative basis of what one might get with the hatch mount popular with some Mudders? My numbers weren't so great there.

The power numbers listed are the "forward" power.


conclusion: with such large variations in SWR, it's probably worth checking one's preferred location to avoid bad surprises and to maximize the power out...






edit: as pointed out by Kofo below, I inverted the gains for the 2 antennas. Obviously, the 1/2w should be the 3.0 . Sorry.

 

[Kofoed]

I didn't read your chart at first, a little too much to digest on an early Sunday morning.

First, on lines 3 and 4, the gain amounts are reversed. The gain listed also over a 1/2 wave dipole.

Second, looking at the SWR readings for the various antenna placements, shows how bitchen the 1/2 wave antenna is. The SWR is not dependent on a counterpoise/"ground plane" nearly as much [actually not much at all] as the 1/4 wave.

Third, it shows what a HORRIBLE place the arb bumper mount is. The SWR of the 1/4 wave is too high for longevity of the radio (unless the whip drastically shortened). Think of all those guys with the 1/4 wave cb antennas mounted there

The 1/2 wave is consistently within or near the advertized spec of 1.5, EXCEPT when the roof rack mesh or bumper bar metal is in the path of the signal...Nobody uses 144 to 145 on FM, show shorten the whip.

The only mounting location for th 1/4 wave that is at or below the advertised 1.5 SWR is below the roof rack--no doubt some odd interaction/interference with all the mesh along the whip (where the SWR for the 1/2 wave is high). But hay, the numbers don't lie--try checking it again with all the gas cans up there

 

[e9999]

yup, sorry, got the gains reversed.

If that wasn't obvious, what I meant by the 1/4 wave under the rack was that the mag mount was on the roof metal with the whip sticking out vertically through the rack mesh floor (without touching the rack of course).

Yes, the 1/2w is sure easier to please than the 1/4w.
The ARB bull bar mount was not too bad for the 1/2, in fact.
Although I didn't try to see if there would be interference from the engine electricals when up there.

Keep in mind also that this was all with a mag mount, and that the latter is what probably caused the large variations since the capacitive coupling through the mag mount likely affects significantly the impedance. Still I'm thinking it could be interesting to measure the SWR with the rear hatch mount discussed before on the forum, esp. with the 1/4w.

 

[Kofoed]

I would add that the 5/8 wave antennas run into the same problem with the need for an effective counterpoise.

I'm telling youse guys, the 1/2 wave solves at lot of problems concerning SWR and not needing a counterpoise (aka "ground plane").

 

[e9999]

well, that 1/4w is quite attractive for compactness and especially how flexible it is, so great for cramped quarters wheeling in principle. Too bad there is only a couple of places with half decent SWRs for it on my truck. Have to say I didn't expect that...

the variation across the band is also interesting, I think. Shows clearly that a fixed length is only a compromise if one is to travel across the band.

I did ask a knowledgeable guy about the SWR values and their effect on the radio, and he wasn't too worried. Was saying that modern radios have good protection against poor SWR. That true?

 

[Cruiserdrew]

I would add that the 5/8 wave antennas run into the same problem with the need for an effective counterpoise.

I'm telling youse guys, the 1/2 wave solves at lot of problems concerning SWR and not needing a counterpoise (aka "ground plane").

Well, sort of. While a 1/2 wave does simplify the mounting options, but it really only gets any gain with good counterpoise. It is healthier for your radio since the antenna tuning for low SWR is much simpler. Looking at Eric's numbers, I would not run that shorty without a better mount. edit-though Eric your friend is correct about output transistor protection, most of the time.

On my FJ40, mounting a 5/8 wave antenna is a major problem, but I got excellent performance with a 5/8 wave mag mount, mounted to the top of my ARB refrigerator! I did switch to a 1/2 wave antenna (pics in another thread) just for this reason.