Bias-t & LNA

[Pape]

Hello RR,

Currently having a Stridsberg 4 ports multicoupler and looking for a upgrade. Since I have 75' of lmr400 on a discone I will go with the LNA at the antenna and ZSFS-8-43 for the coupler.

For the LNA I'm still looking around, ZX60-P103LN+ is interesting but I'm also wondering if you have any though on the laNA ?
Reason for the laNA is the integrated bias-T. While on the subject of bias-t any recommendation ?

Thanks

 

[Ubbe]

Bias-T isn't critical in any way for scanner frequencies. It's just a standard choke coil, that can handle the current, from the power supply that stops the lowest frequencies used from being attenuated and a RF capacitor to the receiver to block the DC voltage and still pass the lowest frequency. You can look at the Nooelec diagram how it's done. The capacitor can be small in the pF range to make the signal have the desirable EQ to slope or be flat and have a 50 ohm resistor to ground to the output to the receiver. Build that in a metal can with either connectors on it or just coaxes coming out from it with connectors on them.

The ZX60 should be possible to open up and add a choke coil to its output to power a 5V voltage stabilizer and then to the power input. Power the bias-T with 12V from a split connector from any scanner.

That Nooelec looks ok but the ZX60 using PGA103+ are little better in its specification. You can't do much wrong with a bias-T and PGA103+ amplifier design so pick anything from Ebay or Amazon. If you are receiving GHz frequencies then it becomes more critical what components to use as it can self oscillate and have other undesirable negative issues at those microwave frequencies.

/Ubbe

 

[prcguy]

I have a number of expensive bias tees but have been using these cheap ones from China, which work great. RF Biaser Bias Tee 10MHz-6GHz DC blocker Coaxial feed FOR HAM radio RTL SDR | eBay

I had a few projects that needed more current and found a version that handles 1.5A but lost the link. That one cost a little more but was still super cheap compared to Minicircuits or other US versions. You might consider a higher current version for things like switching a RF transfer relay when power is removed which bypasses the LNA or filters, etc. When bypassed you can then transmit on the antenna.

 

[Ubbe]

You might consider a higher current version for things like switching a RF transfer relay when power is removed which bypasses the LNA or filters, etc. When bypassed you can then transmit on the antenna.

If building anything yourself, or modifying, I would add such a relay to bypass the amplifier if it goes south. I looked at relays and below GHz all types can be used and there are those who needs much less than 20mA to engage. There's a risk that the gap between the relays connectors are not big enough and a 20dB amplified signal might leak over from the output to the input of the amplifier. Then one might need to use two relays, one as close as possible to the input and the other at the output to separate the signals more, or try and find a relay that have a bigger gap between its contacts, that usually involves a higher current needed to move its relay connectors that much more, so basically the same current are needed anyhow.

/Ubbe

 

[Pape]

Building the bias-t did not cross my mind but maybe it is not that far from my current skills.
Thanks both of you for the input, probably more to come

 

[Pape]

I got the V1 up there, it is currently built from a Lana that was in my part bin, a FM block band, the Bias-t and the ZFSC-8-43+ proposed by prcguy.
My current path lost is estimated at -12DB @ 100 and -15DB @ 1000.
My principal monitoring is 120 ~ 200 MHZ simple FM and 700 ~ 900 Digital trunk

Initial observation:

The FM band stop is a requirement when using the Lana
120 ~ 200 mhz range sweat spot look to be 7 to 10 DB amplification. Anything lower deteriorate the audio quality.
700 ~ 900 mhz range only require some adjustment. It is interesting to note I can monitor some of the system with the Lana power down, only require to increase the dongle gain.

Side note: nooelec 820t2 V4 are really good compared to the E4000 offering from the same vendor.

For the V2 I’m currently looking at the RFBay LNA-1520, low noise figure reasonable IP3 and no attenuation required to get in the sweat spot.
Still investigating RF bypass switch, did find commercial one but the price tag and the insertion lost are just too much for me.
@Ubbe When you talk about relay, you are talking standard audio relay for example the one found in stereo amp ?

 

[Ubbe]

@Ubbe When you talk about relay, you are talking standard audio relay for example the one found in stereo amp ?

I looked at Mouser and other places for the specs of ordinary relays and all seems to have materiel used that can handle up to GHz frequencies when it comes to low level receive signals. The best would of course be to have a metal screened relay.

/Ubbe

 

[Pape]

I looked at Mouser and other places for the specs of ordinary relays and all seems to have materiel used that can handle up to GHz frequencies when it comes to low level receive signals. The best would of course be to have a metal screened relay.

/Ubbe

Since I used a plastic box I should enclose the relay circuit in a metal one. Correct me if I'm wrong but shield does not require to be isolated in this arrangement and therefore I should only concern me with coax center lead ?

 

[Ubbe]

Just put the coaxes in parallel besides each other and solder their center leads, maybe an inch long, to the relay then solder their braids together without melting their plastic isolation. I would use thin coax like RG174 RG316. Buy pigtails with the correct connectors at their ends and cut their coaxes and splice.

/Ubbe

 

[prcguy]

This is a coaxial RF transfer relay and what you want for bypassing an LNA. One connector goes to the antenna, one to the radio, one to the LNA input and one to the LNA output. Its very low loss to 3GHz and runs off 12-15VDC. You can run 12-15VDC up the coax using a bias Tee then use a 3 terminal linear regulator for the LNA if it takes 5V, etc. With power off its straight through to the antenna, power on will power up the LNA and switch it into the circuit.

These cost hundreds of $$ new and this one is new old stock for $29. I just bought one myself for a future project.

Dynatech T4-416D101L 12VDC SMA Female Coaxial Relay Switch | eBay

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[Ubbe]

This is a coaxial RF transfer relay and what you want for bypassing an LNA.

How does the controller pins work? It seems to be a latching relay function, that you have to use the control pins, and not a fail safe relay?

/Ubbe

 

[Pape]

Indeed, I think Ubbe is right. @prcguy let us know if it the case.

 

[prcguy]

Indeed, I think Ubbe is right. @prcguy let us know if it the case.

Unless specified for another voltage the two extra pins should be a normally open pair for an indicator function.

 

[Pape]

Does make senses. This spark all sort of idea that include Arduino and multiple antennas....

 

[prcguy]

Does make senses. This spark all sort of idea that include Arduino and multiple antennas....

For two antennas you can use a 2-way switch like this right at the antennas.

Dow-Key Microwave 401-420832A-2 Radio Frequency RF Coaxial Relay Switch 12VDC | eBay

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Or for multiple antennas a multi position switch like this.

Transco 82152-146c70600 Switch-rf XMSN Line Sp6t 28vd for sale online | eBay

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The 2-way switch listed is 12VDC and can use a bias tee for power where the multi position switch would need a dedicated multi conductor cable. They can be had in all kinds of configurations and voltages. These switches show up all the time for various prices and I usually wait until the 2-way types get down to about $20, sometimes with free shipping.

 

[Ubbe]

I now see on the relay that it says FSCM 50667 that must stand for Fail Safe and then something about Continuous Mode and not latched?
It's hard trying to find any specs, like how much current it uses and how it operates.

/Ubbe

 

[prcguy]

I now see on the relay that it says FSCM 50667 that must stand for Fail Safe and then something about Continuous Mode and not latched?
It's hard trying to find any specs, like how much current it uses and how it operates.

/Ubbe

I have some similar relays and they can draw between 150 and 200ma @ 12V.

 

[Pape]

Unless specified for another voltage the two extra pins should be a normally open pair for an indicator function.

therefore +A and -B are the power and 1 and would be the indicator?

 

[prcguy]

therefore +A and -B are the power and 1 and would be the indicator?

The model in question is different than what I expected. Its 12VDC between the A+ and B- pins then its latching with a momentary 12V on pin 1 or 2. Typically when they are advertised simply as a 12V transfer relay the extra pins are a set of contacts for indicator lights, etc.

 

[Pape]

I have zero continuity between A and B on my side. Also look like it is epoxy sealed. Hard to say but from what I see from my tests I think my unit is bad.