Electronic Circuits and Projects Forum

8th November 2006 11:08 AM

Hi guys,

just a newbie, and wouldnt mind attempting a little project that would be of interest .... i have searched the web as it seems from corner to corner and cant find any diy electronics sites specifically for this or infact for the cb users.

What i would like is any help or suggestions or circuit diagrams or if ofcourse its possible to be done ... a circuit which would allow me to build a small (simple) yet powerful RF amplifier to receive better signal on my uhf tranceiver.

Ofcourse it must allow me to transmit as per normal while connected .. i.e receive better RF signal or db gain and transmit as per normal operation.

is this in any way possible to be done ??

many thanks in advance

Peter

8th November 2006 11:20 AM

UHF isn't simple to build, nor cheap - by far the most effective method would be to use a better aerial - a decent size Yagi will give you 10dB gain on both transmit AND receive, and at UHF frequencies you can get higher gain than that quite easily, because of their small size.

However, you may need to check the legality of it?, often you are limited in what you can do.

8th November 2006 11:34 AM

Thanks for your prompt reply Nigel.

Yeh i suppose a good base aerial will be great as high up as possible, however the tranceiver is in my car which doesnt help a lot.

i thought there would have been something that could have helped get a stronger signal through standard aerial and tranceiver, similar to a lot of amplifiers used for TV uhf signal boosting.

in regards to legality im not really sure ?? but what im after here is greater signal strength and range "in" and not "out" so i couldnt see much problems with that?? unless someone else can shed some light on that topic.

many thanks

peter

8th November 2006 11:40 AM

Amateur radio operators buy or build UHF amplifiers with pre-amplifier. The power is increased on transmit by the amplifier - when not seeing a transmitted signal it switches to the receive mode and amplifies the incoming signal.

The schematic diagram for the amplifier or preamplifier will appear quite simple but as Nigel suggests, it won't be easy for a beginner.

You need to be licensed for the band/mode/power. You also need to know that amplifiers can amplify the stuff you want and stuff you don't want. They can also generate out of band emissions. All stuff that you must manage.

I'd recommend you heed Nigel's advice - taking it a bit further. Take a close look at the whole system to see if you are suffering any losses that can be addressed such as a lossy cable, mismatch, etc.

8th November 2006 12:10 PM

Also bear in mind that UHF is 'line of sight', so range is fairly low unless you are on the top of a hill.

8th November 2006 12:21 PM

Hi Steve,

many thanks for your reply and advice also ..

The set is installed well and all is operational ok for local activity as per normal .. however it would be great to hear on those further weaker signals aswell.

i suppose i just thought i could have dived in a project to play around with and put little extra kick in my signal "in" on my uhf tranceiver.. i know licenced amateur radio kick in amplifiers however they are for both TX and RX signals but i dont want any change to the transmit side.
There are certain restraints even for licenced amateur radio operators of amount of amplification for the TX (depending) on frequency however i didnt think this would have any problems for the RX as there is no transmit just receive.

Many thanks

Peter

8th November 2006 12:34 PM

No, there's no problem increasing the receive sensitivity - you need a suitable preamp that's automatically bypassed when you transmit, either by pin diodes or relays.

However, at VHF and UHF frequencies the limiting factor is the noise of the front end devices, unless your existing front end is particularly noisy it's unlikely a preamp will make very much difference. You also tend to get the problem that you can hear them, but they can't hear you - it's important to try and keep both transmit and receive reasonably balanced.

But such a device isn't simple to build, UHF is very tricky - I would suggest trying looking round for a commercial unit you could buy?.

8th November 2006 12:47 PM

If it's any help?, here's a UHF (432MHz) preamp from the '1976 RSGB UHF/VHF Manual' - if you're serious about wanting to try and built one, I would suggest getting a copy of either the RSGB or ARRL Manuals.

8th November 2006 12:51 PM

Hi Nigel,

Thanks once again for your advice ... its sounding as a not possible at every reply

... i suppose it didnt harm to ask though ...

i found a circuit diagram for an antenna booster which says can boost your signal in for wide range of frequencies however wont suit my application because i want to still be able to use my tranceiver as normal (ie still be able to transmit )

can you give me your thoughts on below and recommend any modifications to allow this or make suitable ??

http://www.uoguelph.ca/~antoon/circ/AA7/activant2.html

many thanks

peter

8th November 2006 01:11 PM

You don't want a receiving amplifier that boosts a wide range of frequencies unless you are very far away from civilization. All the high power TV stations and cell phone towers will overload it.

Electronic Circuits and Projects Forum

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Better db gain for UHF cb

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