Better db gain for UHF cb

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peter_au_75

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

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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 ??

**broken link removed**

many thanks

peter
 
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.
 
peter_au_75 said:
can you give me your thoughts on below and recommend any modifications to allow this or make suitable ??
Click to expand...

As Audioguru says, you don't want wideband - it's much too much trouble, and doesn't give very good gain.
 
At DC and low frequencies things like lead length, parts placement, circuit board traces cause relatively few problems. As the frequency increases even the minor inductance or capacitance in a circuit board trace becomes significant. The construction of the device, at UHF is as important as the circuit design itself. That also suggests that the connections to/from the device require attention. Many of the DIY folks (many are hams, some are not) have contributed to publications that help with all of the issues. As Nigel suggests, consult RSGB or ARRL publications, among others. You may find a preamp that is worth trying.

Keep in mind that TV type pre-amps cover a fairly wide frequency range. Many of them are low in cost and are DC powered. Even if you got your hands on one that was AC powered you could try it out at home to see.
 
I tried a cheap TV preamp on my cable TV system and got 78 stations all at the same time on all the channels. It was extremely overloaded.
 
The TV preamp will boost anything that comes to it to greater or lesser degrees based the specific characteristics of the preamp - it's ability to pass or not pass, amplify or not amplify the signals at the input. The particular amp that I have has a trap for FM broadcast frequencies which eases some of the overload potential.

An improved antenna should deliver increased signal levels as well. The antenna might or might not be selective - though I'd expect greater selectivity as compared to a broadband amplifier - TV or otherwise. The challenge, as Audioguru mentions, is to not overload the front end of the receiver. The potential for that is related to receiver design, the RF environment where you live or drive. If you had one handy the effort would be minimal to give it a try.
 
peter_au_75 said:
Hi guys,

i found details of a FM linear amp on
https://www.electronics-lab.com/projects/rf/013/index.html

any suggestions, or modifications to make this work for 477MHz ??? is this similar to what would be required to help me out?? ...
Click to expand...

That's for a transmitter at VHF, although the circuit itself could be similar at UHF, everything else would be different - every component, and probably the layout as well (it's far more critical at UHF).
 
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