Antenna extended all the way for good SWR?

[SideSide]

Hello all. I will try to keep this simple and short
Transit connect LWB van (2005) using a tri-mag mount, with Sirio 3000 and baby Midland 77-104 UK 27/81. I decided to SWR the new mount and antenna, as you do, and found that it was higher on channel 1 than 40. So, whip needed to be lengthened. By the time I had finished, i had got a really good, very low SWR, but I had extended the whip it to its maximum! (Quite a way - nearly 2 inches possibly) But It is still gripped by both grub screws - just. This just seems a bit odd to me. I rechecked the SWR on two other (cheaper) meters, and it was basically the same reading. Any thoughts? Also, the power reading on my Avair AV-201 is reading about 5 1/2 - 6 Watts across the band. The CB has been realigned and serviced professionally, and the power output is the correct 4W. I'm very confused. Is this normal behaviour? Or is it just a crap meter? it cost about 60 quid, so would've thought it would read better than that. Or, could it be the tri-mag mount?
Cheers for any help or reassurance as ever people,
Chris.

[andy@thunderpole.co.uk]

Hi Chris,

As long as the SWR is good I wouldn't worry too much about the length of the whip.

If you are reading a high power reading it could be due to the impedance of the antenna setup, the only proper way to get an accurate power reading is to use a 50 ohm dummy load plugged into the antenna socket.

Andy

[SideSide]

Hello Andy,
Thanks sonmuch for the reply. I was trying to tell myself that if the SWR value is good, it is what it is. The power reading has me puzzled though. I am using 50 Ohm coax that came with the Tri-mag (Typical RG58) so there must be a build up of resistance somewhere? Is that correct? I'm struggling to work out where this may be. Or, possibly, is it because it is a small van, and running the coax across the roof, then through the door shut, loop through the steel cage behind the seats, then down the front A-pillar to the dash, where my rig is mounted on the windscreen? Could that possibly be causing some sort of resistance?
Sorry Andy I do run away with myself sometimes, please forgive me I realise that there could be any number of causes for the high power rating. So I think I will buy myself a 50 ohm dummy load to play with. I have no doubts that the CB is doing 4 Watts only, as it was Richard on YouTube that did the alignment. (If you know of who I speak?)
Thanks again Andy, I will leave you in peace again
Chris.

[andy@thunderpole.co.uk]

Although they are both measured in ohms, impedance and resistance are different, impedance can be affected by the capacitance and inductance of a circuit too, so although the cable may be 50 ohm there could be other factors affecting the impedance of the circuit.

The only way to get a true 50 ohm impedance and get a true power measurement is to use a dummy load.

Andy.

[SideSide]

Yes, of course it is. Sorry Andy. I understand.
I presume this higher power reading on the meter, possibly due to high impedance, won't be doing any damage to the output, or "finals" of the rig? I only say because I am so confident that the rig is outputting only 4 watts.
Chris.

[andy@thunderpole.co.uk]

Hi Chris,

I wouldn't worry about it if you are sure the output is 4 watts.

Andy.

[SideSide]

Thanks for all your help and reassurance Andy.