UHF aerial mounting

[Lagerfan]

So we've had a recent thread about where to mount the UHF inside but what about the aerial outside? Bullbar is the most popular by far it seems but since we don't have one I was thinking about one of these: Prestigecom.net.au - Gutter Mounting Bracket - $24.00 (http://www.prestigecom.net.au/Gutter-Mounting-Bracket) or even on the roof rack.

Interested in experiences with this kind of setup, I can see problems with clearance but for outback touring this kind of height would be a benefit right? Drilling holes in the roof doesn't really appeal ...

[superquag]

Great idea, had one on my Pajero. Worked very well indeed with a suitable 'Ground - Independant' antenna. Carried the 'Benelec' brand, and looks identical to your link. Price was similar IIRC.

You can set the east/west angle on the front/rear Allen screws.

Never a problem with the gutter bending or being deformed, - just don't get too enthusiastic with the size of whip... A two metre tall solid stick IS stretching the design envelope a bit...

My aerial was around a metre or so, spring base/base-loading coil. Then again, with the Paj's steel roof strength was'nt an issue... :angel:
My door-seal (used RH rear door) coped with the cable and did'nt leak that I noticed. Or heard.

In fairness, the same setup on my Classic has'nt damaged the coke-can roof...:p

James in Gosnells
'95 Classic with working EAS...and leaky sunroof. :mad:

[Leroy_Riding]

The higher the better for the Aerial, within practical reason that is.

In the past I have had all mine mounted to the roof rack, including 6ft 27mhz whips! which can make some servo's nervous when its whipping around their exposed fluro's! the best thing I did was carry a spanner in the door the right size to take the whips of when needed for low clearance situations. (I had a 4.5ft 6db 477mhz, 6ft 27hmz, 4ft tv uhf[for the ex] and a 2ft 3db 477mhz) all on the roof with 4 spotlights, one reverse light and a side light each side. . was murder in cables!)
the extra distance I was able to communicate was worth the trouble IMO but these days with high gain aerials and 'fold down' brackets life would be nice and simple to mount a 4ft high gain fiberglass aerial to the roof rack/gutter and simply fold it down horizontal when not in use or when coming into a car park.

also look at mounting to the spare tire, I have done this before with a 'mast' to get the aerial base up to around roof line so its not transmitting into the side of the car. . .

my 2c

Leroy

[Tote]

The best performing antenna I have ever had on a vehicle was a gutter mounted 3db steel whip on a ground independant base.
generally speaking a higher positioned lower gain antenna will out perform a bullbar mounted higher gain unit.
Roof racks are OK as long as the antenna is ground independant. Otherwise you are back to drilling a hole in the middle of your roof as we did in the good old days.......

Regards,
Tote

[Leroy_Riding]

The best performing antenna I have ever had on a vehicle was a gutter mounted 3db steel whip on a ground independant base.
generally speaking a higher positioned lower gain antenna will out perform a bullbar mounted higher gain unit.
Roof racks are OK as long as the antenna is ground independant. Otherwise you are back to drilling a hole in the middle of your roof as we did in the good old days.......

Regards,
Tote

most are ground independent these days though as you can never rely on even a bull bar to be making good ground contact anyway with thick powder coat. . .
even if not, solder a wire to the base and run it to an earthing point on the car = done, no?

Leroy

[Tote]

The ground plane of the antenna determines the radiation pattern of the antenna but more importantly mounting it higher puts a good ground plane under the antenna from the vehicle's bodywork ( unless you drive a Corvette)From this (http://www.k0bg.com/myths.html) site:

We can reduce the ground losses in a mobile installation by increasing the mounting height of the antenna. Two things happen when you do. First, the resonant frequency increases, due in part to a reduction in the capacitive coupling between the antenna, and the surface under the vehicle. The ground losses decrease, as does the input impedance, basically for the same reason. The reduction in ground losses effectively increases the antenna's efficiency—a worthy endeavor! Incidentally, this is why good installations require antenna matching networks (antenna input impedance less than feed line impedance), and poor ones typically do not.

The another common myth is the level of distortion in the radiation pattern caused by the body of the vehicle. Yes, the pattern is distorted, but not nearly to the level most folks believe. Regardless of the aforementioned shortcomings of modeling software, they're fairly accurate in modeling the radiation pattern. In fact, they fairly mimic empirical testing. That is, if folks are willing to go through the necessary 200+ machinations to describe the vehicle's superstructure to assure even a modicum of accuracy. If you do the tedium, you'll discover the differences are seldom more than about 3 dB. However, the difference may be somewhat greater when modeling antennas mounted low on the back of vans and SUVs. I might add, if the modeled (or real world) measurements exceeds ≈6 dB, then a higher, less lossy mounting location and/or style is in order.

There is a related myth which needs to be dispelled. That is, that ground conductivity in areas near the ocean account for increased propagation and signal strength, and even lower angles of radiation. The truth is, the affect is largely a result of a clear horizon unencumbered by structures, and flora, albeit with a slight decrease in ground losses. Again, localized ground losses have no measurable affect on the radiation angle or (the) far-field pattern!

Advisory Comment: As mentioned above, when an antenna is mounted low to the surface the vehicle rests on (trailer hitch mount for example), a goodly amount of the return current is forced to flow through the lossy surface under the vehicle. If we mount the antenna higher on the vehicle, and place as much metal mass (directly) under it as we can, more of the return current flows in the body of the vehicle. This reduces, but does not eliminate ground losses. Higher mounting typically results in a 3 dB to 5 dB increase in field strength; an obvious worthy goal.

Basically get the antenna higher and it will work better, whether the antenna is in the middle of the roof or the side will have less effect than getting it elevated. The description above is more focused on HF frequencies but similar principals apply at UHF. The ground plane does not have to be a continuous sheet of metal either. If you look at antennas with elements designed to provide a ground plane then these can be rods or a mesh so a roof rack would provide a beneficial ground plane even if it was mounted on a Jeep with a FRP top.

Regards,
Tote

[Lagerfan]

So I'm sold on the idea of gutter or roof mounting but then comes the issue of how do you cable it up? Enter this little item which looks like a neat solution for those of us wanting an antenna option up top: Rear CB aerial mount Pillar side mount 3/8 stud 5M cable Land Rover Defender 90 | eBay (http://www.ebay.co.uk/itm/Rear-CB-aerial-mount-Pillar-side-mount-3-8-stud-5M-cable-Land-Rover-Defender-90-/200941621386?pt=UK_Mobile_Phones_Communication_Rad io_Antennas&hash=item2ec90dd08a) apart from the 3/8 stud mount, but one of these would fix that I guess: 3/8" STUD MOUNT FOR HF ANTENNA AND CB RADIO AERIAL WHIPS | eBay (http://www.ebay.co.uk/itm/3-8-STUD-MOUNT-HF-ANTENNA-AND-CB-RADIO-AERIAL-WHIPS-/190794238615?pt=UK_Mobile_Phones_Communication_Rad io_Antennas&hash=item2c6c390a97).

Shipping might be an issue but if this is a goer I'll drop them a line and see.

[130man]

Hi All, with regard to easily lowering the aerial, I am using marine folding aerial brackets from BCF. At around $14 they are affordable and well worth investigating. I have two arranged to fold sideways across the vehicle , My aerials are on the tray headboard, one each side. It is only a moment's effort to fold them down for car parks, overhanging trees etc. No tools required as they have quarter turn levers built in. I had to fabricate some mounting plates to adapt each aerial to the base but that was not hard to do. Cheers, 130man.