Thread: Sideband CB (27MHZ) Radios Making a Comeback - Cheap Long Distance Comms

Like I said, I accept that newer formats are technically better, and if I was crossing a desert solo I'd probably take a sat phone, but I think there is a place for these units as a secondary fit.

Anecdotal feedback suggests that there are several channels which are monitored by the devoted band of supporters, hence making this at least no worse than having no SSB unit, and in all likelihood better and more helpful for the odd time you are stuck miles from nowhere with no other comms.

Add to that more free channel space over a less densely used spectrum, and the ability to patch into coastal boating frequencies, and I think it's an interesting and rewarding secondary radio.

I also agree with the sentiment that they offer interest and a measure of exclusivity to a club like ours - Defender members especially generally tending towards being a) more technically minded b) less inclined to go with the flow and c) looking for ways to improve our vehicles and have a bit of fun.

I'm willing to give it a shot.

Originally Posted by Sprint

yes, but if you were in the FFR they tend to be mounted in, whats the chances of getting stuck?

given the amount of extra gear that I'd take, pretty good.

I was meaning limited to the uhf/am band radios in this case.

but a good HF set with a good antennae set is very hard to beat.

BTW...

want to know what happens when a rushlish and english speaking radio operator stumble onto each other frequencies on an exercise?

Originally Posted by d@rk51d3

Just looked up my book............. and you're correct.

Am (full band) 4-5 watts.
SSB (upper or lower band) 12 watts.

Learn something new every day.

Finding a good mobile antenna is the big problem.
This is the rig I have: UNIDEN GRANT 40 CH. AM SSB CB RADIO - CAR 4WD HOME BASE - eBay CB Radios, CB Radio, Radio Equipment, Electronics. (end time 15-Mar-10 05:50:14 AEDST)

I still have one of these. Nearly sold it a couple of years ago just to have one less box of stuff. I might get it and the 1/4 wave out again!

Hmmmm must have been a different radio that I learnt about all those years ago.
At full modulation the carrier in an AM signal requires two thirds of the power but conveys no information. The second side band can be viewed as rebundant (overlooking frequency-selective fading in an ionospheric transmission path, that may distort one side band at times). Interference between several carrier frequencies, resulting in steady audio whistles or 'beats' is another disadvantage of AM.
Power may be saved and the band occupied by an AM signal in the frequency spectrum can be halved if only one side band is transmitted without carrier. The result is single side band suppressed carrier signal, called simply single side band signal (SSB ) transmission.
The carrier must be reintroduced at the receiver in such systems and closely adjusted to the original carrier frequency to avoid signal distortion. The introduced carrier carrier must be within 10 or 20 Hertz of the original carrier frequency for adequate intelligibility of voice signals, and stable oscillators are needed for generation of the local carrier.
For SSB the transmitter does not need to generate carrier power, and ratings are in terms of peak-envelope-power (PEP), the power capability at the peak of the modulating signal with linearity of the amplifier is maintained. For equal information content, and 100% modulation, the SSB signal requires only 1/6 th power of the double side band signal. However, the situation is even more favourable to SSB when speech is transmitted. Speech is not a continuous sine wave, and its average power is low with respect to its peak requirements. A peak-to-average power ratio of 10:1 is often assumed for speech, and under that condition, a Double Side Band (DSB ) AM signal would require 1.05 times carrier power, whereas for equal intelligibility the SSB signal would require only 0.05 units of power or 1/21 as much.
Because of the lower power rating, circuit components designed for SSB equipment can be smaller and lower in cost.
For generation of a modulated signal without carrier, a balanced modulator is used. A filter then discards one side band.


And as far as more channels go,I could swear that my 27Mhz radio has 40 channels which just happens to be the same amount as my 476/477 Mhz radio has.But then again I have UHF and VHF as well as HF radios that have variable VFOs and no channels as such,just an oprerating frequency.


And I am sure that it takes no more "Theoretical radio knowledge" to operate a 27 Mhz HF radio than what it does to operate many of the complex controls for setting sell calll and tone control etc on the average 476/477Mhz CB radio.


And yes I have been around radios for longer than I care to think about,back when valves were king,especially on the amateur bands,and way before the Johnson Viking was available with its pll circuitry,we were using cb's that were crystal locked,to set frequencies,but like most things,there were ways around this as well.
And the main reason that 27Mhz cb was all but abandoned by the government,in favour of the 476/477Mhz UHF was NOT for better use of the spectrum to allow more channels,BUT because of the interference generated across the spectrum in the second ,third and 4th harmonics,causing problem with industry and much of the electrical equipment that was coming onto the market,and also to sensitive electronics found in Hospitals etc.But this sort of interference still goes on!!!!! Ever wonder why you have to turn your phone off in a Hospital?


And by the way,if you really want efficiency,you should check the VSWR on your UHF antenna and feedlines,as there is nothing magically different,and no different rules apply just because of the different operating frequency.


Wayne
VK2---
exVK0---





[QUOTE=DeanoH;1180749]
To use a 27 Mhz CB effectively you needed to have some theoretical knowledge of radio transmitters. When you transmit on AM you actually transmit three signals. The carrier frequency, the carrier frequency plus speech (USB and the carrier frequency minus speech (LSB. This is very wasteful of power and bandwidth as all the information is in either sideband. So if you can suppress the carrier and delete one sidband you can put all of your legally allowed power into one sideband and have a much better signal. This is why the power allowed on SSB is higher than AM. The actual radiated power (AM and SSB is the same. Unfortunately all it needs is one person nearby to use AM at the same carrier frequency as you and your mate on the other sideband and you're both gone.
UHF FM was a far better idea. Better use of spectrum allowing more channels,

Still got a 27meg set in the defender, but no aerial at the moment. Only 27meg AM though.


Martyn

Originally Posted by LowRanger

And as far as more channels go,I could swear that my 27Mhz radio has 40 channels which just happens to be the same amount as my 476/477 Mhz radio has.But then again I have UHF and VHF as well as HF radios that have variable VFOs and no channels as such,just an oprerating frequency.

for a standard off the shelf unit, yes but within the upper and lower limits of the band you can operate where you want if your unit has the ability. for the other bands that you're required to be licensed to use you need to stay in the "steps"

Originally Posted by LowRanger

And I am sure that it takes no more "Theoretical radio knowledge" to operate a 27 Mhz HF radio than what it does to operate many of the complex controls for setting sell calll and tone control etc on the average 476/477Mhz CB radio.

For LOS or NOTH whip to whip yes, setting up to do darwin to adelaide though.......

I thick the kiwis still play with am radios as they work ok in the mountains

Originally Posted by Bushie

Still got a 27meg set in the defender, but no aerial at the moment. Only 27meg AM though.


Martyn

You and me both


And funnily enough there's still quite a few people that I do trips with that still have one in their vehicle