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Variable gain preamp anyone?


Tom Crosbie G6PZZ
 

I have always been aware of overloading receivers since my first comms RX, a Drake SSR1.

I think that any RF “gain” control should also include a degree of attenuation in its range.

Chris, I’m wondering how effective a variable voltage PSU (say 0-15V range) will be with the new + amp?

Experiments with a cheap Chinese amp module and variable DC supply kit proved less than successful. I think mainly due to the stability (or lack of!) of the supply.

I was hoping the voltage display could give some correlation to gain. The PSU used an LM317 but the 5k pot was a very course adjuster.

I’m trying again with an LT1083 which is quite a high current device but I’ve bought some 10 turn pots, looking for finer control of the voltage adjustment.

Looking for inspiration, ideas or being told I’m mad…

 

Tom G6PZZ

 

 


Chris Moulding
 

Hi Tom,

I've not tried the Loop Antenna Amplifier + with a variable power supply so I'm not sure how much gain variation you would get.

It's now on the list of things to do next week.

Regards,

Chris


Ray G3NKL
 

I've run my LA+ from both 5 volt and 12 volt LM78xx regulated supplies, and 12 volts definitely gives superior performance in terms of gain.

Ray, G3NKL


Tom Crosbie G6PZZ
 

Hello Chris,

I appreciate the design problems you were faced with in choosing 5V as a source for a Bias-T PSU and the performance jump when using 12V. It’s quite a dilemma as generally the quest is for as much gain as possible and best 3rd IP. I would contend that in real world SWLing, that the maximum performance isn’t always necessary, especially with a very strong adjacent channel which is always in the passband of the amplifier.

 

I was wondering if varying the voltage and thus the gain could be turned into a feature. Could this help in optimising the gain throughout the receiver?

 

Tom G6PZZ

 

From: CrossCountryWireless@groups.io <CrossCountryWireless@groups.io> On Behalf Of Chris Moulding
Sent: 25 September 2020 20:08
To: CrossCountryWireless@groups.io
Subject: Re: [CrossCountryWireless] Variable gain preamp anyone?

 

Hi Tom,

I've not tried the Loop Antenna Amplifier + with a variable power supply so I'm not sure how much gain variation you would get.

It's now on the list of things to do next week.

Regards,

Chris


Simon
 

I have tried this before..12-4v via lm317.

From 12-( circa) 5.5v no real difference in gain, prehaps imd.

From 5.5-circa 4v it does make alot of difference. Switching off circa 4v at t.

This was with the loop amp sold by ccw about 5 months ago..


Tom Crosbie G6PZZ
 

Thanks for your input Simon.
Can you share your LM317 circuit please? My kit was based on a so-called reference design but it had stability problems. I'm wondering if it was wrong components or my soldering or both.
Tom G6PZZ

-----Original Message-----
From: CrossCountryWireless@groups.io <CrossCountryWireless@groups.io> On Behalf Of Simon
Sent: 26 September 2020 15:40
To: CrossCountryWireless@groups.io
Subject: Re: [CrossCountryWireless] Variable gain preamp anyone?

I have tried this before..12-4v via lm317.

From 12-( circa) 5.5v no real difference in gain, prehaps imd.

From 5.5-circa 4v it does make alot of difference. Switching off circa 4v at t.

This was with the loop amp sold by ccw about 5 months ago..


Simon
 

Remind me after dark Tom

Currently up a 9m pole in the rain and wind fixing my fan dipoles..bloody up/down rope “ welded “ itself to upper pulley!

Bit busy trying not to fall..

Typed this on gnd!! Lol


Chris Moulding
 

I just want to raise a point about using the Loop Antenna Amplifier + with a variable supply.

It will be possible to reduce the gain by reducing the voltage applied to the amplifier.

The reduction in gain below 5 V is caused by the bias voltage dropping to a barely workable level.

The amplifier would be prone to generating cross-modulation and intermodulation at that point and I wouldn't recommend it.

For the best RFperformance run the amplifier with a bias-tee at 12 V and add an external attenuator between the amplifier and the receiver if you want to reduce the overall gain.

Regards,

Chris


Simon
 

Or even better still, depending upon frequency of use..( and in conjunction with reducing gain on rx.)

This will not work if you want dc-light but say just want 10-30mhz..make loop small.
Want dc-2mhz..make loop bigger.
Reduces gain where not needed..

Etc..

To remove nasty imd’s from strong mw stations add a notch filter inline with coax.

But yes Chris, I run my loop amps at 12v..(13.8v.) Fully agree with your post.

Simon


Tom Crosbie G6PZZ
 

I’m doing that already Chris with my current amp. Not one of yours, just some cheap Chinese lump hooked up to a longwire.

My attenuator is quite a lump and I was hoping to replace it with something that takes up less shelf space.

Hopefully I’ll be ordering a CCW unit quite soon and start experimenting with loops.

I’ve also got some material that’s just slightly weirder than a bicycle wheel to use as a loop that I don’t think that has been used before.

Three decades ago, after solving an engineering problem, a G4 told me I was a first class bodger and it pleases me to be still living up to that reputation after all this time!

 

 

From: CrossCountryWireless@groups.io <CrossCountryWireless@groups.io> On Behalf Of Chris Moulding
Sent: 26 September 2020 17:11
To: CrossCountryWireless@groups.io
Subject: Re: [CrossCountryWireless] Variable gain preamp anyone?

 

I just want to raise a point about using the Loop Antenna Amplifier + with a variable supply.

It will be possible to reduce the gain by reducing the voltage applied to the amplifier.

The reduction in gain below 5 V is caused by the bias voltage dropping to a barely workable level.

The amplifier would be prone to generating cross-modulation and intermodulation at that point and I wouldn't recommend it.

For the best RFperformance run the amplifier with a bias-tee at 12 V and add an external attenuator between the amplifier and the receiver if you want to reduce the overall gain.

Regards,

Chris


Tom Crosbie G6PZZ
 

FWIW, my interests are in broadcast DXing. MW, HF, VHF, DAB and TV in that order. Not much going on with the last three due to lack of antennas and in about a year or so I'm going to be in a position where I'm going to be forced to relocate. So I need to rethink my interests. I'm looking for compact antennas that I can use indoors hence the renewed interest in loops.
I can't see a loop/amp combo working from 80-900MHz.
But thanks to everyone for the advice and input.
Always appreciated!

Tom G6PZZ

-----Original Message-----
From: CrossCountryWireless@groups.io <CrossCountryWireless@groups.io> On Behalf Of Simon
Sent: 26 September 2020 18:21
To: CrossCountryWireless@groups.io
Subject: Re: [CrossCountryWireless] Variable gain preamp anyone?

Or even better still, depending upon frequency of use..( and in conjunction with reducing gain on rx.)

This will not work if you want dc-light but say just want 10-30mhz..make loop small.
Want dc-2mhz..make loop bigger.
Reduces gain where not needed..

Etc..

To remove nasty imd’s from strong mw stations add a notch filter inline with coax.

But yes Chris, I run my loop amps at 12v..(13.8v.) Fully agree with your post.

Simon