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Joined: January 31st, 2014, 11:16 pm

June 7th, 2018, 11:44 pm #1

Purely theoretical you understand, as I know that amplification above 4w is illegal (for me without a licence any how) ...... but if you were to wish to connect an amplifier alongside a multimode radio and use it from the back of a car (standard 12v Car battery) with a mobile set up, how is it all connected?

A multimode radio on TX pulls from the battery and I presume that the Amp pulls a lot too. Would you connect them separately to the battery? Can you connect them through a double cigarette lighter adapter through the standard socket? Will the Amp drain the battery at a much faster rate than the Multi mode on its own? Would you use a separate battery?

Any top tips or ideas on how I would theoretically wire up and use an amp and a multimode in a car?

Thank you.

Maybe tomorrow I'll find my way home....
CTX new boy.

Joined: January 16th, 2011, 9:03 pm

June 8th, 2018, 5:52 am #2

Expensive i know but when i go portable i use two separate leisure (hi amps) batteries for my hf radio and amplifier which are in the boot and left open for ventilation and last for a good few hours before being charged at home but for mobile i have run separate cables thru the bulkhead and connected onto the battery for both the amp and radio using battery terminal adaptors (just look online)
My cigarette lighter socket has a lower fuse rating than i need for my radio so would blow the fuse...
Last edited by Cozzmik on June 8th, 2018, 5:53 am, edited 1 time in total.


Joined: April 25th, 2018, 12:36 am

June 8th, 2018, 8:54 am #3

It seems a lot of mobile amps come with quite short stubs of not very thick power cable which you'll need to extend. The current-carrying ability of cable is related to its cross-sectional area. 0.5mm cross sectional area stuff is generally what you see on power leads for 4W radios, usually 0.75mm for higher power rigs and multimodes and 1mm is acceptable for amplifiers around 100-150W. But take those as a minimum and use the thickest you can work with, because the voltage drop between the battery and the equipment will be higher with thinner cable. Cigarette lighter connections aren't really much good for anything, but you'd get away with it for a 4W radio on its own - and nothing more!

How much current you draw from the battery will vary with different rigs and amplfiers, but you can slightly overestimate that a rig putting 7W into something like a Midland 747 amp produces 70W output at a total current drain of no more than 15A combined. For a long run from a battery, I wouldn't really want to use 1mm cable even though it is electrically safe. I use a lithium-ion golf caddy battery, 22A/h, and it comes with a lead ending in Anderson Powerpole connectors, and so I use Powerpoles on all my gear.

I made up a splitter lead out of some Powerpoles, 2mm cable and 30A chocolate block connectors to allow me to connect 3 devices to the battery. But I haven't actually had to get the linear out of the bag yet because I am blessed with a couple of great portable locations where 20W FM and 25W SSB makes the trip to almost everyone I can hear. For a car, I'd definitely want a separate battery for the radio setup to the car battery because you can draw a lot out of a car battery running higher power static mobile after a while. Plus if the budget stretches in the end to a lithium battery for the radio stuff, you'll find that technology maintains its voltage better as the battery runs down than lead-acid batteries, and your rig and amp will thank you for the extra volt (or more).

CB manufacturers are all optimistic about what their amplifiers can really do. The Midland 747 mentioned above is sold as a "100W max" amp with a max 10W input, but to save the transistors you wouldn't want to run it much above 70W FM from about 7W input, or 80W SSB. The failings of amplifiers on SSB come in two kinds. One is that not many mobile amplifiers actually produce linear output, but the RM KL203 and Midland 747 both seem OK. The other failing can be that if you drive an amplifier all the way to the manufacturer's stated maximum, it will either sound horrible because really you're overdriving it, and it can be generating lots of crap on other frequencies besides your desired signal! It's good practice to run any Midland/CTE/RM/Zetagi amp at no more than about 75% of the stated maximum either for input or output power. I'm pretty sure none of the mobile 100-150W amps on the market have proper low-pass filtering on their outputs, so an external LPF is also a good idea if you can. A lot of good radio sites also have other radio services up on towers that you really don't want to be splattering with harmonics etc.

This is a conservative approach to doing it and a lot of people I'm sure get along quite happily without being so cautious.

TC62 26CT2817
Yaesu FT-891 CRT SS7900 Harvard H-404 Uniden 400 Icom ICR-75 Amstrad CB901 President Grant Baofeng UV-5R KW Electronics KW202

Joined: December 17th, 2015, 6:56 pm

June 8th, 2018, 9:37 am #4

These are my pull figures for a KL203 coupled to a CRE8900 for portable on a strong constant whistle, SSB. Ie not scientifically measured but was good enough for what i wanted to know.

Radio Input Radio Draw 203 Draw
5Watt 3.2Amp 6.5Amp
7.5Watt 3.8Amp 9.7Amp
10Watt 4.3Amp 10.7Amp

Dont do FM so no idea how that affects things.
Last edited by Smudger on June 8th, 2018, 9:38 am, edited 1 time in total.
CT1938 / FOX INDIA 420

Joined: December 26th, 2010, 8:08 pm

June 9th, 2018, 9:28 pm #5

Hi , when I go portable I use an Acom 1000 amp, run it from a generator. been using it for around 3 years now and never have a problem. If you go down the generator route, get an inverter type that is friendly for laptops etc.
It aint cheap but is safe to use with all your radio stuff and laptops, Honda if you can afford.

Andy 591/P
Designing and building even bigger Antennas. Hoping for some good DX, waiting for solar minimums to fade away.