Loop Antenna Amplifier - Service Bulletin 4 - HF performance improvement


Chris Moulding
 

We have had feedback from customers that the performance at HF of the VLF/HF version of the Loop Antenna Amplifier falls below other similar loop antennas.

I've taken advantage of the Christmas break to investigate the problem.

The inductance of the antenna loop used with the Loop Antenna Amplifier can have significant reactance at higher HF frequencies that can affect the impedance match to the amplifier.

For example the inductance of the 3m wire supplied with the amplifier is 3.7 uH when set up as an inverted delta. The inductance of a 622 mm diameter bicycle rim is 0.6 uH.

I've run some tests where I incorporated the antenna element into a low pass filter so that the effect of the inductance on the impedance match to the amplifier was eliminated.

This is an identical technique to the termination resistor using wirewound resistors where the inductance is used in a low pass filter.

http://www.crosscountrywireless.net/rf_load_with_wirewound_resistors.html

The modification is very simple consisting of removing the common mode choke (red and black wires) between the PCB and the M6 stainless steel terminals on the head unit. A direct connection is made between one of the terminals and the ground plane on the PCB. The second connection is made via a 2.2 uH inductor between the amplifier input terminal and the other M6 stainless steel terminal. An 82 pF capacitor is connected between the terminal end of the 2.2 uH inductor and ground.

With the 82 pF already on the later boards or added to early boards this makes a 4th order low pass filter. It also acts as an effective low pass filter before the first amplifier reducing VHF FM broadcast signals by 40 dB.

The performance of the combined antenna loop and modified Loop Antenna Amplifier is significantly improved above 10 MHz and it should now be comparable at HF to other loop antennas.

Here are two photographs of modified head units using through hole components and surface mount components.





Regards,

Chris


Paul Gulliver
 

Hi Chris,
Slightly confused, are you saying the component rising from the pcb in the bottom photo is an inductor - to me it looks like a carbon film resistor - It's a long time ago (over 20 years) since I bought any electronic components and I certainly wouldn't have any inductors like that.
Would it be possible to make a self supporting 2.2uH inductor from (say) 22swg tinned copper wire - if so approx turns and dia would help - I have no means of measuring inductance

Paul


Roger Need
 

If you remove the common mode choke won't this affect the loop balance and reduce the depth of the nulls you get when rotating the loop?

Roger


Chris Moulding
 

Some of the through hole inductors look just like resistors.

I wouldn't recommend using a self supported coil as the magnetic field wouldn't be contained in a ferrite core and could potentially cause a feedback loop or reduce the isolation of the other common mode choke on the PCB.

According to the toroid inductance calculator two turns round the toroid core used in the now redundant common mode filter should give 1.76 uH and three turns 3.96 uH. I've not tried this as a solution yet but I will tomorrow.

Another high quality alternative would be to wind 24 turns on a T50-6 toroid core. This would give a very low loss 2.2 uH inductor.

Regards,

Chris


Paul Gulliver
 

Ok, I'll have a go at the 2 turns on the redundant toroid - tomorrow or Sunday - the amplifier is just stuck in a draw at the moment so anything is worth a go.

Paul


Chris Moulding
 

As the loop is now correctly matched and is a very low impedance the common mode choke separating the amplifier ground from the coax cable ground isolates the loop (and amplifier) from the feeder.

It doesn't need the extra common mode choke to get the balance and null depth.

I've confirmed this with tests on the modified amplifier.

Regards,

Chris


chipbutty
 
Edited

I appreciate the information but most of this goes straight over my head. I don't understand the technical side of things when it comes to antennas. I just wanted a loop that performed well on shortwave but I find myself using the MLA-30 because it performs better. I can solder fine but I would need really explicit instructions to perform this mod.

Chris, you also offered a swap for the HF/VHF unit as this would perform better on shortwave with the supplied coax. Would I be better off performing this mod on my current VLF/HF unit or swapping for the HF/VHF? I'm competent at soldering and enjoy it but only with painting by numbers instructions! I would really like to try this mod on my current VLF/HF unit if it improves performance.

Cheers,

Rob


Chris Moulding
 

Rob,

You would be better off doing this modification on the VLF/HF unit than changing it for the HF/VHF version (unless you wanted VHF coverage).

Over the next few days I'll make a video showing how to do the modification in great detail.

I'm also looking at the possibility of offering a modification service so that customers can send back their Loop Antenna Amplifiers for the full set of modifications to the latest specification..

I just need to check one more suggested change in the workshop before offering that service.

Regards,

Chris


Paul Sayer
 

Not having another loop to compare it to I have no idea how well mine is working. 

Paul G0VKT 

On Sat, 28 Dec 2019, 15:00 Chris Moulding, <chrism@...> wrote:
Rob,

You would be better off doing this modification on the VLF/HF unit than changing it for the HF/VHF version (unless you wanted VHF coverage).

Over the next few days I'll make a video showing how to do the modification in great detail.

I'm also looking at the possibility of offering a modification service so that customers can send back their Loop Antenna Amplifiers for the full set of modifications to the latest specification..

I just need to check one more suggested change in the workshop before offering that service.

Regards,

Chris


chipbutty
 

Excellent Chris. I'll look forward to the idiot proof video ;-)


Paul Newland
 

Just the ticket Chris 
Endowed with no technical knowledge in this field and completed with a full set of thumbs, I would take advantage of this offer if it comes to pass.
Best Wishes
Paul



On Sat, 28 Dec 2019 at 14:00, Chris Moulding <chrism@...> wrote:
Rob,

You would be better off doing this modification on the VLF/HF unit than changing it for the HF/VHF version (unless you wanted VHF coverage).

Over the next few days I'll make a video showing how to do the modification in great detail.

I'm also looking at the possibility of offering a modification service so that customers can send back their Loop Antenna Amplifiers for the full set of modifications to the latest specification..

I just need to check one more suggested change in the workshop before offering that service.

Regards,

Chris


chipbutty
 

Paul - You can pick up the MLA-30 very cheaply. I paid £25 including postage. It makes for a nice travel loop as the element collapses down nicely. I'm currently away for Christmas at my father in laws and have it setup on the windowsill. Little video here from the other day https://youtu.be/y-f6EDpJ-zQ Chris's loop is equally as portable. It's the element I guess which dictates portability.


On Sat, Dec 28, 2019 at 03:39 PM, Paul Sayer wrote:
Not having another loop to compare it to I have no idea how well mine is working. 
 
Paul G0VKT 

On Sat, 28 Dec 2019, 15:00 Chris Moulding, <chrism@...> wrote:
Rob,

You would be better off doing this modification on the VLF/HF unit than changing it for the HF/VHF version (unless you wanted VHF coverage).

Over the next few days I'll make a video showing how to do the modification in great detail.

I'm also looking at the possibility of offering a modification service so that customers can send back their Loop Antenna Amplifiers for the full set of modifications to the latest specification..

I just need to check one more suggested change in the workshop before offering that service.

Regards,

Chris


Eric Inloes
 

Hi Chris,

Will these modifications ship with all orders going forward?

Thanks! 


Chris Moulding
 

Yes, all new Loop Antenna Amplifiers will have all the modifications.

Regards,

Chris


Paul Gulliver
 

This is my mod (see attachment), I done it yesterday before I read Chris's offer to do the mods himself - hope I've done it correctly, looking at the pads at the top, the left one goes directly to one of the loop connectors, the centre pad is one end of the inductor and the right hand pad is one end of the capacitor. The remaining ends of the inductor and capacitor connect to the 2nd loop connector.

Couple of points to note, I have no means of checking component values, the capacitor is marked 82pf but it's over 30 years old and may have a very wide tolerance and the inductor is 2 turns on the "redundant" toriod core .

I haven't done any tests yet, later today all being well.

Chris: in view of your offer to carry out the mods, to save both of us postage on the complete unit, would it be possible for you to pop an inductor and capacitor (leaded varieties not surface mount) in the post, I can then be sure of using the correct values.


Cheers,

Paul


Chris Moulding
 

That looks like a neat modification.

As soon as the New year holiday is over I'll order a batch of wire ended inductors and capacitors so that I can ship them out to customers on request.

I'll also make a video on how to do the modification with these new parts when they arrive.

Regards,

Chris


Paul Gulliver
 

I've just completed some tests on my setup, the results are not very good but bear in mind I'm not using the recommended components as I mentioned in my last post.
A quick update of my setup, a 1.1m dia loop from 8mm dia copper, using the CCW bias tee for all tests. Receiver is a RSP1A with a 10db 50 ohm attenuator on the input (as recommended in a previous post for matching purposes). Software is SDR Console and WSJT-X for the digital mode. Loop orientation set for max signal in the NE/SW direction and the base of the loop is 2-3m above ground.

I done the test at three frequencies:

1) Shannon Volmet on 5505KHz  Signal to noise ratio CCW amp  30,   MLA-30 amp 65-70

2) DDK3 Hamburg Weather fax station 7780KHz       SNR  CCW amp  45-50,   MLA-30 amp  55-60

3) FT8 digital mode on 14074 KHz - for each amplifier I measured the number of messages received over a period of 6 minutes and recorded the farthest CQ msg received
   CCW amp - 147 messages , max distance 1400 miles  --  MLA-30 amp - 675 messages, several stations between 3000 to 4000 miles

I've attached the waterfall from the WSJT-X program which shows the marked difference in traffic between the 2 amplifiers.

There was a gap of approx 10 minutes between the tests of each amplifier so propagation shouldn't have changed much.
Not a scientific test but as good as I can get to a practical test, I'll do the tests again when I get the correct components from Chris.

As it stands the MLA-30 is still coming out on top.

Cheers


Paul


Brent Mcl
 

So the loop i just bought not even here yet needs to be updated ?


Roger Need
 

Paul Gulliver wrote
7:27am  


A quick update of my setup, a 1.1m dia loop from 8mm dia copper, using the CCW bias tee for all tests. Receiver is a RSP1A with a 10db 50 ohm attenuator on the input (as recommended in a previous post for matching purposes).
You don't need the 10 db attenuator on an RSP product because it has a 50 ohm input. Chris was referring to the Airspy when he suggested this.  You will see your signals go up on the RSP by 10 dB when you remove it.

Chris does not publish his design but most active loops have about 25 to 30 dB of gain.  You can use less than that but then the receiver will probably need more sensitivity in order to have the same Signal-to-Noise ratio.

Roger


@sdoros
 

I have a RSPDuo and a lot of noise from FM stations when I use loop antenna amplifier from 1.8Mhz till 18Mhz. I have tried feeding the amp via bias tee and via indor unit. Also tried with an external FM notch filter alonf with RSP fm filter. No change, noise is still there.
Any recomendations how to vanish this noise?