Midland 98 Plus CB - advice on installation, please

[Snagger]

Hi folks.

I have just fitted a Midland 98+ to my Land Rover, using a standard antenna mounted to the roof rack. The radio has an SWR gauge connector port, but I don't have a gauge. Is it important in setting the unit up?

As for the antenna itself, I take it that running an earthing wire from the base of the antenna to a good earth on the vehicle is all that is needed?

Sorry if they daft questions, but this is my first radio.

Nick.

 

[gary in ohio]

You need to check the SWR, but from what I saw in the manual the external meter doesnt provide SWR just signal strength.. THe correct SWR is important to ensure the most power from the radio goes out the antenna. You should borrow or purchase an SWR meter. They should be pretty cheap at most radio shops.

 

[SunTzuNephew]

Hi folks.

I have just fitted a Midland 98+ to my Land Rover, using a standard antenna mounted to the roof rack. The radio has an SWR gauge connector port, but I don't have a gauge. Is it important in setting the unit up?

As for the antenna itself, I take it that running an earthing wire from the base of the antenna to a good earth on the vehicle is all that is needed?

Sorry if they daft questions, but this is my first radio.

Nick.

Is your vehicle negative earth? If not, don't install that radio!

 

[PhulesAU]

if you are having trouble finding a meter, check around for HAM operators in you neighborhood.

 

[Snagger]

Thanks for the replies so far.

The vehicle is negative earth - no need to worry about that!

I have a local Maplin shop which will stock a SWR meter, but I can't see how to adjust the SWR (and I don't know what SWR is, anyway). The only controls on the unit are the PTT switch on the mic, the on/off/vol knob, the squelch knob, the two channel selector buttons, a button to swap between EU and UK channels and a scan button... This is all pretty alien to me.

 

[SunTzuNephew]

Thanks for the replies so far.

The vehicle is negative earth - no need to worry about that!

I have a local Maplin shop which will stock a SWR meter, but I can't see how to adjust the SWR (and I don't know what SWR is, anyway). The only controls on the unit are the PTT switch on the mic, the on/off/vol knob, the squelch knob, the two channel selector buttons, a button to swap between EU and UK channels and a scan button... This is all pretty alien to me.

You adjust the antenna length to adjust SWR (normally)...by moving the antenna element (unless it's a long whip on a ball mount) up and down just a bit in the socket it rides in.

Th meter should have two sockets on it, and is inserted into the antenna feed line. You may need a short jumper of coax with connectors.

Set the channel to about the midpoint frequency, close the doors and measure the SWR - if it's within 1.3:1 or so you're in good shape, if not move the antenna up or down a bit (experimentation and reiteration needed) to get that SWR. Then check the low and high ends, make sure you're still within 1.3:1 or so. Then remove the SWR meter and have a pint.

 

[Snagger]

Fantastic - that makes sense now (I understand the principles of radio and the need to optimise the antenna length to match a harmonic of the wavelength).

The antenna has a hex bottom that screws into the mounting, a coiled section above that to let it bend for obstructions, and just above that is another hexagonal section roughly 2" long. there appears to be a recessed screw in one of the flats of that hex piece - I take it this is where you adjust the antenna length? Presumably we're talking about adjustments in the 32nds of an inch, not 1/4"?

You recommend an SWR meter that fits in the antenna cable, not one that plugs into the small SWR port at the rear of the radio (like a small phone or mic jack port), then, or does it connect to both simultaneously while setting it up?

 

[PhulesAU]

yes, get an in-line meter. you will usually remove it after setting the swrs. the shorter the antenna the more sensetive it is to adjustment. A little bit goes a long way. Here's some generalized info for Ya.

http://www.firestik.com/Tech_Docs.htm

 

[SunTzuNephew]

Fantastic - that makes sense now (I understand the principles of radio and the need to optimise the antenna length to match a harmonic of the wavelength).

The antenna has a hex bottom that screws into the mounting, a coiled section above that to let it bend for obstructions, and just above that is another hexagonal section roughly 2" long. there appears to be a recessed screw in one of the flats of that hex piece - I take it this is where you adjust the antenna length? Presumably we're talking about adjustments in the 32nds of an inch, not 1/4"?

You recommend an SWR meter that fits in the antenna cable, not one that plugs into the small SWR port at the rear of the radio (like a small phone or mic jack port), then, or does it connect to both simultaneously while setting it up?

That bit with the recessed screw sounds like the adjustment, and I'd get the inline meter...

Cheers!

 

[gary in ohio]

Thanks for the replies so far.

The vehicle is negative earth - no need to worry about that!

I have a local Maplin shop which will stock a SWR meter, but I can't see how to adjust the SWR (and I don't know what SWR is, anyway). The only controls on the unit are the PTT switch on the mic, the on/off/vol knob, the squelch knob, the two channel selector buttons, a button to swap between EU and UK channels and a scan button... This is all pretty alien to me.

The SWR (Standing Wave Ratio) is a Ratio of transmitted power from the radio (forward) and Power reflected back from the antenna due to poor antenna )reverse). A perfect SWR (or often called match) is 1:1.. In reality you never get 1:1 but very close. SWR meters are powered from the RF power coming down the the antenna cable. Some SWR meters will read both forward and reverse on the same meters. Other require you to flip a switch to switch between them.

 

[Snagger]

Guys, thank you very much for your patience! I did a google search on SWR and found a few explainations and instructions on setting it up, but your help seemed clearer, so even though I should have done a search earlier, your answers are not wasted!

I have attached a couple of photos of the antenna mounting to make my question a bit clearer, but one of those internet instructions (from a CB retailer) mentioned that the antenna should be mounted with the coil as far away from metal objects as possible. Now, I got the impression that this was some sort of winding to increase antenna gain (which is not fitted on my antenna), not the coiled spring at the base of the antenna, but I'd love some clarification on that.

I'm concerned because (as the photos show), that coiled spring is sitting right between the load level and upper guard rail of my steel roof rack and not far from the stowed steel wheel chocks. The upper hexagonal section connector from the thick gauge spring section of the antenna and the thin gauge, stiff, straight section is level with the rack's guard rail. If I understand the antenna theory correctly, the fit should be alright, though perhaps not ideal. I can move it if necessary - I have plenty of co-ax left and the mounting merely clamps to the roof rack. What do you guys think?

 

[SunTzuNephew]

That coil (loading coil) is designed to make the antenna electrically longer - a 1/4 wave for typical CB frequencies is around 9' long and by winding the signal around a bit the antenna 'looks' long to the radio signal. (This is a oversimplification but gets the point across, I think).

Move it as far away from other metal as you can. If you can't get a good (better than 1.5:1) match, try moving the antenna.

Sometimes all you can do is what you can do, so if you can't get better than 2:1 or so, try another antenna and mount.

 

[gary in ohio]

Move that mount UP so the coil is above the metal rack. I would also put the antenna on the inside of the rack to keep branches from grabbing the antenna and coax. Also PL259's are NOT waterproof. I would seal that connections once your happy with the location and the SWR is good. There is a product called coax seal that will work or shrink wrap tube filled with silicon will do the trick.

 

[Snagger]

Right, then; I'll move the antenna to the middle of the front section of the rack, midway between the spot lights, and fit the clamp around the rack's guard rail to raise the whole lot up.

That should also prevent any impairment of the omni-directional signal - I read that fitting it to the side of the vehicle like that may reduce the range on one side (the open side, with the side on which the rack is in relation to the antenna being better).

I had already used some heat-shrnk tube over the joint of the cable and connector and was going to use Vaseline on the threads to keep the water out - the Vaseline should be waterproof but should also allow good electrical conductivity actross the threads.

So, just to recap, the coil at the bottom of my antenna is for transmission and reception efficiency and not just a spring to allow the antenna to deflect when it strikes branches and so on?

Thanks for your patience.