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Loop Antenna Amplifier service bulletin 2


Chris Moulding
 

Recently we have had reports of high noise and possible intermodulation problems with all versions of the Loop Antenna Amplifier.

This only seems to occur when used with certain SDR receivers.

Thanks to very detailed reports by customers Paolo Romani and Jarkko Iisakkala I've been able to work out and simulate what the problem is.

Most good quality radio receivers have carefully designed front end filters that match the incoming coax feed from the antenna. It appears that some SDR receivers have very elementary filters and band switching arrangements that do not give a good 50 ohm match to the antenna.

In the worst cases this can lead to such a bad mismatch in the Loop Antenna Amplifier base unit that it can go into oscillation raising the noise floor and causing intermodulation effects.

The simplest way to resolve the problem is to put a small 50 ohm attenuator say 3, 6 or 10 dB on the input socket of the SDR receiver.

This is a good solution for two reasons. All antennas connected to the receiver via the attenuator will see a matched 50 ohm load so it will help improve the signal to noise ratio on all HF antennas connected to the receiver even though overall signal levels are reduced by the attenuation of the attenuator.

The second reason is that the resistors in the attenuator will drain static electricity from the external HF antenna to ground if the coax shield is correctly grounded helping to protect the receiver and the attached computer.

This has been difficult to diagnose as all the receivers and test equipment I use to test the Loop Antenna Amplifiers have a matched 50 ohm input. These include CCW SDR-4+ Special Edition, CCW Sentinel 4, Yaesu FT-817ND, Kenwood TS-430s, Icom IC-M600, TenTec Corsair II, HP network analyser and HP test set with spectrum analyser.

SDR receivers that have been definitely proved to have the poor match problem are Airspy HF Discovery. Other receivers are on the suspect list but I will only add them when I get two or more reports confirming the problem.

Regards,

Chris


Tom Crosbie G6PZZ
 

Chris,

I think you make a very important point here. We are all trying to squeeze, the last drop of signal out of the general mush of atmospherics and active antennas, loop or otherwise, have become part of the DXers arsenal. We also expect much from our receivers too SDR, DSP or analogue, and perhaps “mis-use” them by keeping every gain control at maximum hoping to hear that elusive signal. In my many years of selling receivers and transceivers I would try to explain that gain controls were for turning down and not up!

 

 I have an old active vertical up in the air that doesn’t even have a plug on the downlead yet and I’m hoping for this to go live over Christmas. It is wideband (0.5 to 1500MHz) so will be used with several receivers. I have recently purchased a switched attenuator (0-80db according to the Chinese manufacturer) and it will sit between my antenna switch and receiver selector switch so its available to any combination. I have a longwire too and often reducing the signal level from that allows the receiver to do its job better through the rest of the signal patch.

 

Attenuators – Every home should have one!

 

Tom G6PZZ

 

From: CrossCountryWireless@groups.io <CrossCountryWireless@groups.io> On Behalf Of Chris Moulding
Sent: 12 December 2019 20:53
To: CrossCountryWireless@groups.io
Subject: [CrossCountryWireless] Loop Antenna Amplifier service bulletin 2

 

Recently we have had reports of high noise and possible intermodulation problems with all versions of the Loop Antenna Amplifier.

This only seems to occur when used with certain SDR receivers.

Thanks to very detailed reports by customers Paolo Romani and Jarkko Iisakkala I've been able to work out and simulate what the problem is.

Most good quality radio receivers have carefully designed front end filters that match the incoming coax feed from the antenna. It appears that some SDR receivers have very elementary filters and band switching arrangements that do not give a good 50 ohm match to the antenna.

In the worst cases this can lead to such a bad mismatch in the Loop Antenna Amplifier base unit that it can go into oscillation raising the noise floor and causing intermodulation effects.

The simplest way to resolve the problem is to put a small 50 ohm attenuator say 3, 6 or 10 dB on the input socket of the SDR receiver.

This is a good solution for two reasons. All antennas connected to the receiver via the attenuator will see a matched 50 ohm load so it will help improve the signal to noise ratio on all HF antennas connected to the receiver even though overall signal levels are reduced by the attenuation of the attenuator.

The second reason is that the resistors in the attenuator will drain static electricity from the external HF antenna to ground if the coax shield is correctly grounded helping to protect the receiver and the attached computer.

This has been difficult to diagnose as all the receivers and test equipment I use to test the Loop Antenna Amplifiers have a matched 50 ohm input. These include CCW SDR-4+ Special Edition, CCW Sentinel 4, Yaesu FT-817ND, Kenwood TS-430s, Icom IC-M600, TenTec Corsair II, HP network analyser and HP test set with spectrum analyser.

SDR receivers that have been definitely proved to have the poor match problem are Airspy HF Discovery. Other receivers are on the suspect list but I will only add them when I get two or more reports confirming the problem.

Regards,

Chris


Eric Inloes
 

Hopefully RSP2 isn't on this list. Have you had a chance to test the RSP2 and the RSPdx? 

I do own a RSP2 and a RSPdx soon hopefully (Very difficult to a hold of one right now) and willing to test but haven't purchased a kit quite yet. Been a bit busy. 


John Pfeifer
 

Looking at a couple of BNC attenuators from Mouser per your recommendation. Would either one work or should I go with the higher priced one ?

The first one costs $7.95 and has no info about frequency response: AIM-Cambridge / Cinch Connectivity 27-9300-3 BNC 50 OHM BNC(M) (F) 3db.

The second one costs $21.95 and is flat from DC to 1Ghz: Crystek 549-CATTEN-03R0-BNC 3dB BNC 50 Ohm 2 watts.

Thanks,
John - WL7M

On Thu, Dec 12, 2019, at 11:53 AM, Chris Moulding wrote:
Recently we have had reports of high noise and possible intermodulation problems with all versions of the Loop Antenna Amplifier.

This only seems to occur when used with certain SDR receivers.

Thanks to very detailed reports by customers Paolo Romani and Jarkko Iisakkala I've been able to work out and simulate what the problem is.

Most good quality radio receivers have carefully designed front end filters that match the incoming coax feed from the antenna. It appears that some SDR receivers have very elementary filters and band switching arrangements that do not give a good 50 ohm match to the antenna.

In the worst cases this can lead to such a bad mismatch in the Loop Antenna Amplifier base unit that it can go into oscillation raising the noise floor and causing intermodulation effects.

The simplest way to resolve the problem is to put a small 50 ohm attenuator say 3, 6 or 10 dB on the input socket of the SDR receiver.

This is a good solution for two reasons. All antennas connected to the receiver via the attenuator will see a matched 50 ohm load so it will help improve the signal to noise ratio on all HF antennas connected to the receiver even though overall signal levels are reduced by the attenuation of the attenuator.

The second reason is that the resistors in the attenuator will drain static electricity from the external HF antenna to ground if the coax shield is correctly grounded helping to protect the receiver and the attached computer.

This has been difficult to diagnose as all the receivers and test equipment I use to test the Loop Antenna Amplifiers have a matched 50 ohm input. These include CCW SDR-4+ Special Edition, CCW Sentinel 4, Yaesu FT-817ND, Kenwood TS-430s, Icom IC-M600, TenTec Corsair II, HP network analyser and HP test set with spectrum analyser.

SDR receivers that have been definitely proved to have the poor match problem are Airspy HF Discovery. Other receivers are on the suspect list but I will only add them when I get two or more reports confirming the problem.

Regards,

Chris


Chris Moulding
 

Either attenuator would work especially at HF.

Regards,

Chris


Chris Moulding
 

I don't have an RSP2 or RSPdx to test but it's would still be good practice to add an attenuator for HF use.

The advice from Tom, G6PZZ is very sound.

Regards,

Chris


Paul Gulliver
 

Hi Chris

Is the attenuation value important or just the 50 ohm matching? there are some on ebay with just 1 dB attenuation - would this be OK, or do I need a higher attenuation?

This is for a RSP1A - just thought it would be worth a try

I haven't heard back from you regarding the email I sent a couple of days ago, not sure if you haven't had time to digest the results or your email is playing up again.

Paul


On 12/12/2019 at 20:53, Chris Moulding <chrism@...> wrote:


The simplest way to resolve the problem is to put a small 50 ohm attenuator say 3, 6 or 10 dB on the input socket of the SDR receiver.




Chris Moulding
 

For HF use I would suggest 6 or 10 dB, 1 dB is too small to have any real effect on the matching.

The background HF noise level from most antennas is probably 20 dB above the receiver noise floor so adding 6 or 10 dB will not have any effect on receiving low level signals. Try removing the antenna to see how much the noise level falls on the display to demonstrate this point.

Adding the attenuator allows very strong signals to be received correctly without hitting the limit of the ADC in the SDR receiver.

Paul, Re your email I'm trying to get as many orders as I can out before Christmas so I'll go through my email this weekend.

Regards,

Chris


Guy Atkins
 

This is a low cost step attenuator I recently purchased, to go with the three or four individual "bullet" attenuators I already owned:

https://www.ebay.com/itm/NEW-0-20db-Variable-Step-Attenuator-50-OHM-for-Electronics-Test-Laboratory-Ham/153381181864

When my CCW VLF/HF loop arrives I'll watch for any noise floor/intermod problems on my RSPdx.

73, Guy


Chris Moulding
 

If anyone wants to get their soldering iron out and make an attenuator with three resistors then a simple 8 dB tee attenuator can be made from two 22 ohm resistors, one 47 ohm resistor and two coax sockets.

Connect the two 22 ohm resistors in series between the input and output socket centre pins.

Connect the outer connector shields of the input and output connectors together.

Connect the 47 ohm resistor between the centre of the two 22 ohm resistors and the connector shields.

I'll build one tomorrow and post a photo.

Regards,

Chris


Chris Moulding
 

Today I built a quick prototype of a dual attenuator box. One side is an 8 dB attenuator, the other side is 20 dB.

I've attached a photo to help explain the 8 dB tee attenuator I described yesterday.

With this simple layout it has good VSWR up to 250 MHz and with correct internal shielding added could go much higher.




Regards,

Chris


Eric Inloes
 

Anything new on other suspected SDR with the poor impedance matching issue?

Thanks