18 Wheeler HF Multiband Antenna Options

[KB5TJ]

First, an quick intro -- I've dabbled in Amateur Radio since my initial license about 25 yrs ago, which led to a 15+ year career as a commercial Merchant Marine operator. I'm not a degreed engineer, but I've been around the block a few times in the field, so to speak ;-)

Project is to mount the most efficient radiator, within cosmetic limits, on a semi truck for ops from 20M down to 40M (I'd like the latter to be somewhat better than a radiating dummy load). 80M would be a nice luxury. I own the truck so holes are not a problem.

Transmitter is a Yaesu Ft-857D. No linear. From what I recall and from what I've read recently (I've been out of the loop, so to speak, for quite a few years), the best options are a 102" whip coupled to an FC-40, or one of the full size 'screwdrivers' (a Hi-Q or a High Sierra, the latter being favored due to height restrictions).

A 102" whip I could mount on the side. See photo at http://img.photobucket.com/albums/v288/sparks19/side.jpg

The cab is all fiberglass, with a few aluminum stringers here and there, with an aluminum bottom pan (floorpan) about at the white circle (dot). From what I gather, this would work 20M okay, and 40M somewhat. As to counterpoise, the aluminum floorpan of the truck is roughly 8 ft wide by 10 ft long. Bonding this to the twin frame rails, which are about 23 ft long, probably make a better counterpoise than the average car. Frame rail height above ground is very roughly 2 feet, probably giving me an advantage (capacitance-wise) over the average car. The truck is DC-coupled to the trailer, which is an interesting animal -- 53 foot long, but with mostly a wooden floor (with steel or aluminum side-to-side stringers) and otherwise aluminum sides and roof.

I could also mount a 102" whip on the rear of the cab, but if I were to do that, I might as well mount a full size screwdriver back there (within limits... the Hi-Q would be a bit tall with the recommended 7' 6" whip, but the High Sierra would be no height problem retracted). Again, the rear of the cab is fiberglass, but -- the antenna would be within about 3.5 feet of the 9. 6" tall slab of of the trailer. I could probably tolerate the node in the radiation lobe if the trailer did not induce some kind of impermissible capacitance problem. See photo at http://img.photobucket.com/albums/v288/sparks19/rear_quartering.jpg

So there you have it, folks. I'd rather have the full size 'screwdriver' for better radiation efficiency on 40M, as well as the opportunity to work 80M, but it would have to be on the back of the cab for asthetic reasons. Some things, such as the horizontal 'grab bar', could be reworked to allow the antenna. The tall exhaust stack, essentially DC grounded to the frame, could not. Any type of bonding to the trailer are also not an option (not that I'd think it would be wanted), as the trailer does not belong to me, and is swapped it with others on an almost daily basis.

For a side-mounted 102" whip, I could allow for some type of base loading to make 40M more efficient... given what I consider the optimal base mounting location (just above the aluminum floor pan), I have about 8" of 'slack'.

My conclusion so far is that the full size 'screwdriver' would work better, were it not for the trailer; and if that all things were about the same, the 102" whip and the FC-40 would be the cheaper alternative (altho cost is not really a factor, everything being the same I see no need to go to the extra 'hassle' and cost of a full size 'screwdriver' if a 102" whip and auto-coupler will perform about the same under the constraints noted).

I am also concluding that a 102" whip and auto-coupler would outperform a shorty 'screwdriver' -- such as the Hi-Q and High Sierra ATAS variants.

Intended comms are mostly with other over-the-road drivers for rag chewing and weather reports (including base stations) over a 500-1000 mile radius.  Not looking to be a DX or contest hound ;-)

Thanks in advance,
de Mark :-)

[K0BG]

It gets a lot more complicated than there is space here. Although I haven't completed my article on OTR installations, there is enough data on my web site to get you started.

You used the term counterpoise, instead of ground plane, and although they are similar, they are not the same thing. Quarter wave vertical antennas require a good ground plane under them if any modicum of efficiency is to be garnered. If you were driving a sedan, the best place is in the middle of the roof. Any other location will produce more ground plane losses.

On an OTR semi, the structure is mostly fiberglass these days, and about the only decent place is atop the foot rail attached to the top of the fuel tanks. Even then, on some models, the tanks are enclosed in fiberglass. Since most drivers don't own their rigs, they stop short of drilling holes, and mount their antennas on the mirror brackets. Which, by the way, is the worst possible place with respect to ground plane losses.

Since you can't reduce the losses to any extent, the use of an auto coupler and whip isn't the best idea. If you opt for a screwdriver, use one of the better models. The cheap ones just won't take the vibration. This is especially true on 80 and 40 where the coil is extended out of the housing. The only remotely tuned mobile antenna that doesn't suffer this fate is the HiQ. And you have to think about cap hats too.

As I said, there is a lot more to it than meets the eye.

Alan, KØBG
www.k0bg.com

[KB5TJ]

Alan, thanks for the input. I have delved thru your excellent website and come to the conclusion that all I am going to have for a ground plane will be the aluminum floor of the cab, the frame rails of the truck, and the miscellaneous associated components (engine block, transmission housing, fuel tanks -- which are aluminum, exhaust, etc) -- assuming that all of this is properly bonded together.

The one thing I am unable to determine is which would be the better radiator:

1) a 102" whip fed by an auto-coupler, which could be mounted low on the side of the cab or behind the cab, or

2) a full size screwdriver fed by something like a Turbo Tuner, but which would have to be mounted on the rear of the cab (in rather close proximity to the aluminum trailer).

I understand that if there were no mounting location constraints, the full size screwdriver would be the better radiator.

As to mounting on the fuel tank steps, are you saying that it would be better to get the base of the antenna as low to the earth as possible, even to the point it would be below the 'effective' ground plane of the truck's chassis? It seems like this would be somewhat 'unbalanced' and would tend to slighly increase capacitive loss.