separate antenna inputs for vhf and 800MHz

[n8chb]

I would like to wire my uniden bc796 for separate antenna inputs for vhf and 800MHz
Anyone done this on any of the newer type models?

A few years back it was relatively easy to do but the newer surface mount boards
make it difficult without a diagram.


73,

Roger n8chb

 

[John_M]

Why would you want to do this? Are you getting strong intermod interference?

 

[gr8amp]

There are many reasons to do this. The primary reason is to allow for the use of multiple antennas, each optimized for a specific band. This can be applied to help resolve intermod issues or other interference problems, or simply improve performance in a specific band. For example, to help reduce nextel interference in my receiver I use a carefully placed indoor antenna for 800MHz, and an outdoor scantenna is switched in for the other bands.

Back in the day you could just break the antenna connection to one of the front end bandpass filters, and tag your antenna right there. Unfortunately, with the surface mount components and epoxy covering used today this is nearly impossible.

My solution was to use a cmos RF switch, controlled by one of the control lines that switch the filters in the scanner. This was done to a BC898t, but the same basic idea applies to most scanners. I quickly prototyped this a couple weeks ago, and some pictures are shown below. I am now awaiting PCB's with a more appropriate layout.

If course, these parts are tiny as well, so that doesn't really help out the surface mount problem. An alternative would be to purchase a PIN diode switch from Mini-Circuits, American Microwave, or some other manufacturer.

 

[Freqed]

" You can tell a true Scanner Enthusiasts by where he sits his coffee cup!" LOL Good job on the antenna leads, something some of us newer owners would never think of.

 

[n8chb]

There are many reasons to do this. The primary reason is to allow for the use of multiple antennas, each optimized for a specific band. This can be applied to help resolve intermod issues or other interference problems, or simply improve performance in a specific band. For example, to help reduce nextel interference in my receiver I use a carefully placed indoor antenna for 800MHz, and an outdoor scantenna is switched in for the other bands.

Back in the day you could just break the antenna connection to one of the front end bandpass filters, and tag your antenna right there. Unfortunately, with the surface mount components and epoxy covering used today this is nearly impossible.

My solution was to use a cmos RF switch, controlled by one of the control lines that switch the filters in the scanner. This was done to a BC898t, but the same basic idea applies to most scanners. I quickly prototyped this a couple weeks ago, and some pictures are shown below. I am now awaiting PCB's with a more appropriate layout.

If course, these parts are tiny as well, so that doesn't really help out the surface mount problem. An alternative would be to purchase a PIN diode switch from Mini-Circuits, American Microwave, or some other manufacturer.

That's a good alternative solution.

Didn't want to do it that way but might have to give in.
The epoxy is enough to scare most away and it looks like uniden
might be doing multiple layering on top because there is not much to look
at on the bottom side of the board.

Would you suggest manually switching bands and using my Fluke to find
the test points where the voltage is being switched on the band filters or did
you have to locate them near the VCO's for each one.

Someone asked about why this way and you guessed it. I am using mono band
antennas for each band.


Anywho that's the poop
and good work on your layout om.

73

Roger

 

[gr8amp]

Would you suggest manually switching bands and using my Fluke to find
the test points where the voltage is being switched on the band filters or did
you have to locate them near the VCO's for each one.

This would be the easiest method. You can usually find the signals you need at a row of vias near the antenna connection. These are the lines that control the switching of the bandpass filters. Not all unidens are the same, but I have seen a few with a similar setup. This is what it looks like in the 898t. http://mysite.verizon.net/james_k/BC898t_Bandpass_Select.JPG

You could also hunt around for the VCO select lines. I dont like doing this for a few reasons. First, these wont be as simple to find. Second, there are usually only 3 VCO's in these radios, so that limits your configuration a bit. If you grab the wrong spot (for example...the collector of the vco switch rather than the base) you might run into PLL stability problems, or even load the output of the VCO (this could then lower the LO drive level to the first mixer, which could then cause receiver performance issues)

If you do search for the VCO lines, look for silkscreen lablels "VCO1" , "VCO2", etc. These are "usually" pulled up to the CPU control voltage through a 3.3k -- 4.7k resistor and are active low.

 

[n8chb]

This would be the easiest method. You can usually find the signals you need at a row of vias near the antenna connection. These are the lines that control the switching of the bandpass filters. Not all unidens are the same, but I have seen a few with a similar setup. This is what it looks like in the 898t. http://mysite.verizon.net/james_k/BC898t_Bandpass_Select.JPG

You could also hunt around for the VCO select lines. I dont like doing this for a few reasons. First, these wont be as simple to find. Second, there are usually only 3 VCO's in these radios, so that limits your configuration a bit. If you grab the wrong spot (for example...the collector of the vco switch rather than the base) you might run into PLL stability problems, or even load the output of the VCO (this could then lower the LO drive level to the first mixer, which could then cause receiver performance issues)

If you do search for the VCO lines, look for silkscreen lablels "VCO1" , "VCO2", etc. These are "usually" pulled up to the CPU control voltage through a 3.3k -- 4.7k resistor and are active low.

Thats excellent,

U have given me lots of gud info thanks again.

Abt 20 yrs ago I put together a (low-jack type) RF directional searching kit for a local ham club and the pin diodes generated a lot of noise but we were not interested listening to the audio so we didn't care. I wonder if you have found much noise with just 2 antennas? I had 4 1/4 waves trunk mounted and they were switching at a good speed.

Roger

 

[gr8amp]

If you are wondering about the switches contribution to the receivers overall noise figure, I am unable to provide a quantitative answer. However, given the relatively low insertion loss of the switch (ADG919), I would suspect that this is negligable.

As far as switching noise, I have not noticed any. The switch control line was kept short, and everything is contained within the scanner. I would predict problems if the control line was run externally.

 

[n8chb]

fine on all,

Your way of doing it is looking better all the time because there is no access on the bottom
of the board and the area on top is covered by a shield sealed at one end with apoxy.

I don't know if I'm ready to make a doorstop out of this thing yet. hi

I'll let you know how I make out om.


73,

Roger