Fitting HF aerial

[TheBeastie]

Have just been to sweetie shop and got my first two HF aerials for 20m and 40m (Watson single banders). Am struck by just how massive they are - about 4' rigid and a 4' whip on top!

I have been planning to mount at the front end of the N/S wing and using a sort of aluminium plate to go under the wing instead of the large washer provided to give a bit more support for the mount. I was planning to run the coax through a protective tube, under the bulkhead and straight up through the horizontal bit between the seats and the rear bulkhead, then across to the radio.

I did not want to really go onto the roof since that would make it very awkward to get to when changing aerials (it will also make vertical max height quite worryingly tall!) but if front wing is not deemed safe then will consider. I rejected the idea of a sidewall mount on basis of advice that the weight of the HF aerials would be too significant for the mount.

Two questions before I start drilling. Does this wing location make sense, and am I right in proposing front n/s rather than front o/s?

73

Malcy

[Scotian]

Doesnt make alot of difference which side you put it on. Of course the best location on a vehicle is directly in the middle to get a nice even ground plane around it. If you have it one side or another or front/back you will gat some gain away from the ground plane (this can be good and bad dependingon situation) Having said that for amature radio usage it wont be significant enough to worry about I dont think.

I have two mounts. The 10m rig is running off a firestick on the front right of bull bar and the other mount that isnt being used is on the right side of the body. They both have been pretty much full proof even while driving through thick forrest and they quite happily bend to 90 degrees without any damage as they have spring and rubber mounts.

Both mounts are earthed directly to the body at the closest possbile point and of course the radi is earthed in the same way.

[western]

I'd go to the other end & fit them on right angle steel brackets bolted the the outer ends of the rear cross member & brace the rigid section to the body just below the rear roofline, you might fall foul of the law by fitting them to the front -- as in the antenna could cause injury to pedestrians.

I've seen a few HF antenna on tow bracket mounts with the bracket secured by sandwiching it between the tow attachment & ball hitch with the 2 large bolts through the antenna mount, should be easty to make your own up.

[TheBeastie]

Thanks guys

The idea of going onto the rear cross member is attractive but I had it in my head (probably mistakenly) that it would be better for the aerial to be clear so it can receive better???

The mounts are cheap so no problem getting another if it will work OK and would save drilling a hole in the wing top. The legality of going on the front had crossed my mind since pig sticking pedestrians is not on my agenda

I would need to be careful over bracing since from what I can see the bottom parts of the two aerials are different - one is marked 20m and the other 40m - so I will need to be able to easily change the whole aerial but I am sure I could easily make up some quick release gizmo.

Malcy

[western]

doesn't matter where the base of the antenna is, those are tall enough to be able to transmit/recieve over the roof without any problem from the vehicle. you could use some exhaust type U bolt clamps to support it & protect the antenna lower section with a rubber sleeve or similar.

[Scotian]

If you possibily can, have the base of the antenna clear of anything that its earthed to or you will end up reflecting radiation in directions you dont want it, putting your VSWR off and in general not getting 100% from your set up. But again, its not the end of the world and you will still be able to operate with it not set up perfectly.

[cipx2]

These are base loaded ground plane antennas. The bottom (thicker) part doesn't radiate/receive signals. All it does is compensate for the difference between the 1/4 wave length on that freq band and the actual length of the whip (the top part of the antenna).

My advice is to have the antenna mounted at the back in such a way that the base load will run at least 1 inch away of the body (better 2 inches) and the whip will start from roof level up. You can use the spare wheel bracket, the swing away carrier mounting bolts if you have one, add a bracket/support to where a swing away carrier normally attaches and so on. I would also use a plastic bracket or equivalent clamped to the gutter, with a suitable hole so the antenna can run through it for additional lateral support of the top end of the base load.

[Scotian]

Do you mean somthing like this Barret HF antenna I have here on my old work truck. Yes its mounted exactly like I said it shouldnt but I wasnt in charge of comms on that contract Some fat usless Pr1k was!! (we have history!!, hence why I'm not working just now!!)

And yes of course. With this kind of antenna only the whip needs to be proud of the grown plane to save problems with SWR and radiation reflection causing undesired results.

[cipx2]

Something like that, not the front but on the back. The base load can run along with the body but as soon as the whip starts (or any metal parts acting like holder/spring for the whip) it must be above the roof level.

From the radiation pattern p.o.v, the best place would be in the middle of the roof but then everywhere you go you'll need a 4.3 m clearance and I don't think it's practical.

[TheBeastie]

Thanks guys

Here are details of the aerial off the Waters & Stanton web site. Hope it answers possibilities of whether base part radiates or not.

Seems clear that I need to look at back not front and will investigate the mounting issues further.

WHF-20B Watson Single band HF Mobile Antenna

* 20m Single Bander * Budget Price * 250W * Fibre Glass Helical * Tuneable Whip * Rust Resistant * High Efficiency * 3/8in Stud Mount * Approx. 2.25m long

The Watson WHF mobile series of antennas are of a high quality, heavy duty, slimline construction. They have been purpose made for the HF Amateur radio bands 160, 80, 40, 30, 20, 17, 15, 12, 10, 6, 4 and 2m. They are designed to operate on the phone portion of the band, bandwidth and VSWR will, of course vary according to the mounting location. VSWR at resonance should be 1.5:1 or less when properly matched. The whip is field tuneable for lowest VSWR and double locked with stainless steel grub screws. If you require a greater bandwidth, an ATU may be required. Heavy gauge copper wire is used in the construction of the resonator and this wound onto a fibreglass mast and finished in sleek black. The fittings are chrome and the 1.2m (4ft) long stainless steel whip that fits into the resonator ensures that the antenna not only works extremely well but also looks very smart.

[Scotian]

I agree 100% with what cipx2 just said although personally I think the antenna look a million times meaner and smarter on the front.. but thats just what I'm used to.

Here, this is the kit I'm used to installing. On this page there is a rear mounted antenna, its not a very big pic but you can see it. http://www.codan.com.au/HFRadio/Configurat...54/Default.aspx

[Scotian]

Just did a quick google of that watson antenna.. Cipx2, are you sure its not radiating the full length? It says that is still may require an ATU so I would have thought the whole thing was the antenna with the bottom being a coil and the top being a straight whip. The Codan and Barret kit I showed has a big ATU that the whip screws into.

[cipx2]

Yeap, I'm positive about that. In fact, TheBeastie should have known this as well 'cause this is basic antenna theory one should study for the HAM exam.

Whether is base loaded, top loaded or loaded somewhere in the middle, the loads (which are coils with or without capacitors in a series/parallel combination - same as ATUs) do not radiate. That's in theory, of course. In practice, they do radiate a little bit but it's never taken into account. As soon as there is a fair distance between the load and the body (which is ground) so the parasitic capacitance between them won't detune the base load then all will be fine.

Of course it will need an ATU if you want to sweep the hole 40m or 20m band (only if transmitting - not needed for reception). These whips are way too short to provide a decent bandwidth. Even a full ground plane needs an ATU on those bands. The shorter the radiating part of the antenna - the narrower its bandwidth (VSWR < 1.5).

It's simple to test that. Mount it on a pole in the ground (at least 2 m high), add at least 2 radials 1/4 wavelength long, don't bother to tune it - use a an ATU, put your rig into CW transmit mode (with the proper checking for a clear freq and the proper announcement required by regulations) and do the neon bulb/tube trick to see where it radiates.

That's right, that Barret antenna has something like a built in ATU. I didn't read the specs but it should be similar to what the HAMs call "the screwdriver antenna" plus some form of memory circuit to go back to previous settings for various most used freqs.

[TheBeastie]

In fact, TheBeastie should have known this as well 'cause this is basic antenna theory one should study for the HAM exam.

The Beastie is fully prepared to hold up hand and admit to being a very very newbie on this radio stuff. Yes antennas do come into the exams but at a very rudimentary level.

The guy in the shop who seems to eat sleep and breathe radio did say it would radiate whole length not just the whip which is why I was thinking the front, but perhaps compromising on the back is the route to go.

I will try to digest all the comments this evening when home from work.

Thanks for all the helpful comments you have offered

Best wishes

Malcy