Thread: What happened to your Discovery 2 today?

We must've gone to the same school, Rick.

Originally Posted by PhilipA

From what I have read the SWR is not so important on UHF.

On VHF if the SWR was out a lot , the RF stage of the CB could be damaged but this apparently does not apply to UHF.

Regards PhilipA.

I cannot agree with SWR being “not so important on UHF” and that is because reflected RF is RF that cannot be radiated…. would you be happy with only half of your transmitter power getting out because of antenna mismatch! However, the UHF-CB antennas sold by the various retail chains are sold “pre-tuned” and so the overall performance of the installation rests on decisions made when putting it all together and the feed coax is part of the antenna - which is what kicked off this discussion. PS: My preference is for RFI brand commercial antennas, they come with a cutting chart to tune the antenna in the absence of a SWR meter. Centre of band for UHF-CB is Channel 40 (477.4000Mhz) and the 1/4 wave length is 157mm.

A mountain is far more a problem than SWR on modern antennas.

UHF CBs are basically line of sight and to pretend otherwise is foolish on the users behalf. There is no skip with UHF.

A short low gain antenna has a vertically round radiation pattern some of which may go over the top of an obstacle while a high gain antenna has a low flat trajectory which may travel further on completely flat country but add some hills and no dice.
From his responses Old Bob appears to be an amatuer radio buff, and they can be quite er exacting in their requirements.
Regards PhilipA

Originally Posted by PhilipA

A mountain is far more a problem than SWR on modern antennas.

UHF CBs are basically line of sight and to pretend otherwise is foolish on the users behalf. There is no skip with UHF.

A short low gain antenna has a vertically round radiation pattern some of which may go over the top of an obstacle while a high gain antenna has a low flat trajectory which may travel further on completely flat country but add some hills and no dice.
From his responses Old Bob appears to be an amatuer radio buff, and they can be quite er exacting in their requirements.
Regards PhilipA

Silly me…. Yup it’s those pesky mountains and of course antenna tune has nothing at all to do with effective transmitted power or RANGE, or lossy components (which is where this conversation started)

Any advice for

INJECTOR 1 PEAK CHARGE LONG, I am guessing wiring harness some where. Any one had it ? also got a Generic CAN bus message most likely related. I suspect CAN bus time out as it lost communication.

Water came over the bonnet made it to the other side under full steam then just cut out on me, lucky by then bonnet was out of the water, took a few goes to fire back up.

Originally Posted by PhilipA

BTW on the bullbar most of your signal is going backwards as the ground plane of the bonnet and turret is the main ground plane.

If I were to put Antennas on my bullbar I would fit ground plane independent antennas , so that the signal would radiate more equally.

My setup is a small 4DB ground plane independent fibreglass whip above my drivers door B pillar and that will give a more even radiation pattern and is higher up. It is fitted to a pivoting mount so that it doesn't get hit by the garage door

I have a BIG 8 db helical whip just lying in the back of the car if I am ever out in the desert wanting to communicate . Both whips will just screw into the ground plane and are interchangeable.

I hardly ever have it on now I am not in convoys anymore as I cannot stand the inane and obscene chatter.

Regards PhilipA

Originally Posted by onebob

Hi RRT,
Re: “How would this be tuned per se if the cable simply ran from the antenna to the receiver, other than positioning of the aerial of course”
A common 1/4wave antenna for UHFCB is actually “broad band” and covers from 380 to 520Mhz. They are OKAY in a straight forward installation like a simple cable run between transceiver and antenna. However the 80 UHFCB band is comparatively narrow with the channels ranging from 476.4250 to 477.4125 but the 1/4 wave broad band antenna is able to be tuned to the centre of the band ie ch40 to squeeze the max efficiency out of the meagre 5W we are allowed to transmit. The antenna is tuned by adjusting the length of the radiator, usually by shortening it whilst observing the SWR. Many owners don’t bother with it - some of us do though .
PS: is it an Antenna OR an Aerial? ……. Accepted usage is that an Antenna is used for Transmitting and Receiving and an Aerial is for reception only.

Had a chance today to talk to the bloke I got the aerials from!

Even more confusing but essentially the aerials are 'ground independent' and were why they recommended as I had told him I was mounting them on the bullbar. Certainly Onebob the extra connections were not recommended as to loss. Therefore they do not require a ground plate, but would benefit from the location up high on the roof but the chance of damage is high of course.


Hope I got this right, but the ground independent aerials have the coaxial going up through the aerial and are generally not interchangeable (???????) due to the aerial being tuned by this internal coaxial attachment, and hence are screwed as a unit to the attachment point and care required not to turn the aerial whilst attaching which could damage the coaxial. See the more you get the told the more confusing it becomes...

You can tell if white man speak with forked tongue by looking at the bases.
The ground plane will be a cylinder about 120MM long, then the antenna.
Post a photo and One Bob (or I) will tell you.
BTW , I didn't see that photo of the switch box. That is HORRIBLE..
You will use the short antenna most of the time so just have the cable ending in plugs for the hand held and switch them if you feel the need.
Regards PhilipA

errrr... no .. ?

ground independent doesn't preclude the use of interchangeable aerials(or whips, or randome .. or whatever other you prefer).

I used to run a whip type 4db, but it was too flexible and copped more damage than the replacement GME randome.

Many aerials have screw in fittings where you can replace it with whatever length sensitivity you prefer.

as just one example of this kind of system take a look at GME's AE4702/AE4705 series. They have different model types with various suffixes, but those suffixes really only refer to the colour of the aerials, and maybe the 'completeness' of the product.

GME AE4705BTP as an example: if you follow the link, what you do is mount the spring base to the bullbar antenna mount. The whip section is separate, and is screwed onto the spring base not too tightly, then a grub screw is used to lock it into place.

If you check out the images in the link: You can see the screw thread on the smaller of the two whips(the 2.1dbi) aerial. You don't need to do/undo/mess about with any connections. the connections are in the base and whip, so as you wind the whip into the base, the PL239 connectors mate up.

So in practise what you do is on the drive out to the mountain terrain, you have the 6dbi installed, from Melb to most mountain areas that usually means about 45-60mins of fairly flat terrain .. so 6db gives you just that bit more range.
As you get closer to mountain/hilly country .. pull up, have smoko, fill tanks, slash .. etc... and in a couple of mins you pull out the allen key, undo the grub screw on the 6db whip, unwind it out, wind in the 2.1db whip, tighten up the allen grub screw .. pack away the allen key safely) drive away. Really takes about that time.
Usually takes longer to remind oneself to do it!

You can get similar setups for flexible whip types too.