I had some MPS2222 transistors and built up a quick LZ1AQ and swept it, with the sweeps and picture of the ugly-build added to the Photos. This is only for comparing? with Caaarlo's build and sweeps (thanks, Caaarlo, you did a nice job building yours). His nanovna sweep didn't include gain sweep. I used the DG8SAQ VNWA with the same two frequency sweeps, and with calibrated-out 1:1 input transformer (unbalanced to balanced) which shows both input impedance and gain on the same sweep. I used -30 dBm input level to ensure the loop amp is not overloaded. I'm not sure what transformer and turns Caaarlo used; I used a BN73-302 binocular core with 8/8:6 turns which is what I use with a nominal 50 ohm coax cable output. (Same as what Everett Sharp N4CY's LZ1AQ PCB uses.) The transistors run out of gain-bandwidth product at 30 MHz as you can see from the sweep (the gain is rolling off). Caaarlo's loop amp should have similar gain since both used the same transistors.

/g/loopantennas/album?id=296351??? If that doesn't work, search Photos for "Steve LZ1AQ Sweep".

On Thu, Jul 11, 2024 at 09:41 PM, Steve Ratzlaff wrote:

The
transistors run out of gain-bandwidth product at 30 MHz as you can see
from the sweep (the gain is rolling off). Caaarlo's loop amp should have
similar gain since both used the same transistors.

Steve,

Thank you for the compliments.

I have just measured gain as per your request, and mine is almost flat:

1MHz 33.15dB
7.1MHz 33.51dB
30MHz 33.50dB

At 1MHz I am sure that the lower gain is due to 1uF input capacitors used, which add some reactance hence reducing input level, output level and measured gain. Some 10 or 100uF should lift the gain to the same level as the other frequencies.

Regarding your gain rolling off, it is not a problem of transistors, but a problem of ugly-build as you just named it. For being such a low impedance input, those input capacitors, their legs, and everything in between your generator and the emitters,? are inserting an important amount of reactance in series with the actual input of the amplifier: the emitters.

So you basically have a voltage divider consisting of a large parasitic inductance and the smaller input impedance, reducing the actual power delivered to the input hence reducing output.
Parasitic inductance is not a problem when load impedances are much higher, but when they are as low as in LZ1AQ amp, in the order of 2-4 ohms, they become a serious threat to performance.

A quick calculation:

those extended legs are probably 25mm for each capacitor, total 50mm, that is an inductance of about 52nH

At 1MHz the reactance is 0.3 ohms
At 30MHz it skyrockets to 9 ohms, which is at least twice the resistive component at the emitters.

I would strongly recommend to shorten all input paths as much as possible, because what you are experiencing has nothing to do with the transistors.?

73

开云体育

Hi Caaarlo,

Thanks for your interesting comments. Please tell me the exact setup and equipment you used to do your own gain measurements, and what input level did you use?

Thanks,

Steve AA7U

On Fri, Jul 12, 2024 at 03:14 AM, Caaarlo wrote:

I would strongly recommend to shorten all input paths as much as possible, because what you are experiencing has nothing to do with the transistors.?

73

Hi Steve and Caaarlo,

As an aside.

If you use the same construction technique as shown in the photo that Caaarlo posted.

/g/loopantennas/photo/296346/3804668

Initially cover the whole board in self-adhesive Kapton, and then just cut out areas where you need a ground connection with a scalpel, you will be able to achieve much greater circuit density, and make prototypes a lot quicker than by laying down insulation where it is needed.

This is now my preferred method of prototyping, and I also use thin brass sheet (from model suppliers) instead of PCB material, usually when I need improved heat sinking.

Regards,

Martin

I forgot to add a link to the photo.

/g/loopantennas/photo/267767/3804785

Regards,

Martin

On Fri, Jul 12, 2024 at 12:25 AM, Steve Ratzlaff wrote:

Hi Caaarlo,

Thanks for your interesting comments. Please tell me the exact setup and equipment you used to do your own gain measurements, and what input level did you use?

Thanks,

Steve AA7U

Steve,

My setup is simple:

The VNA is a NanoVNA, which is a self-powered VNA, hence no ground influence, same goes for the laptop connected to it.
1) I did not use any balun or transformer to couple the VNA to the balanced input of the LZ1AQ.
2) The output of the LZ1AQ was also connected directly to the VNA
3) The actual power entering the amplifier is around -30dBm.
4) The turns ratio of the trifilar output transformer is 6-6-5, on a small unknown toroidal core of high ? (Mix 73 or 75).

Some words explaining each point of my setup:

1- No input transformer: First, all the setup is floating (no ground). Second, the input impedance is so low and dominant compared to the possible external couplings and interactions with the generator, the computer and ground (capacitive and/or inductive) that it ensures symmetry by itself.
Adding a transformer is not a free lunch. One has to evaluate whether the balance of pros and cons is beneficial.

2- The output of the LZ1AQ was also connected directly to the VNA, as the output coil is floating, and could be theoretically used with either a balanced or unbalanced load. In reality there is some capacitive coupling of about 20pF to ground from the primary windings that might slightly alter the "floating condition" of the VNA so perhaps a current balun would have optimized the output measurement. But 20pF have a reactance of 360 ohms at 30MHz, more than 2 orders of magnitude higher compared to 1/2 Zin (~3 ohms) that connects the ground of the vna to the ground of LZ1AQ. The influence is minimal.

So. as you can see, I consider that the ill effects of adding transformers, in this case, would outweight the light ill effects of not using them.

3) Input level: The NanoVNA has a fixed level and it is -13dBm within the HF range. But due to the huge mismatch presented by the very low Zin of LZ1AQ, the actual power entering the amplifier is around -30dBm.
This reduction of around 18dB (depends on Zin at each frequency) has to be considered when computing gain.? I have to do it manualy, as the NanoVNA software is quite basic.

4) The turns ratio of the trifilar output transformer is 6-6-5, on a small toroidal core of high ? (Mix 73 or 75) theoretically optimal for 69 ohms, but either 75 or 50 ohm loads can be connected.


73 !!

While replying to steve I realized I might be close to saturating the VNA input. So I will insert an attenuator after LZ1AQ output and see how it goes.

On Fri, Jul 12, 2024 at 06:29 PM, Caaarlo wrote:

I will insert an attenuator after LZ1AQ output and see how it goes.

I just found an old LZ1AQ board I prototyped some time ago using MP222A transistors.

I connected a 20 dB attenuator on the amplifier output, and made a quick 1-50MHz sweep using a LiteVNA64 and internal storage.

/g/loopantennas/photo/267767/3804910

Regards,

Martin

On Fri, Jul 12, 2024 at 08:27 PM, W0LEV wrote:

I have no idea of the IP1 (1 dB output compression power).

Mine is around +23dBm

I also tried varying the sweep power, no significant changes noticed up to the maximum level of 0dBm.

Regards,

Martin

On Fri, Jul 12, 2024 at 08:41 PM, Everett N4CY wrote:

When Sweeping a LZ1AQ amp you need to use a 1:1 transformer between the VNA and the amplifier.

Ideally, but when using a stand alone VNA, that is not connected to anything, it is so small in terms of wavelength, that it is effectively "floating" and balanced without the need for an extra transformer.

This is the big advantage of the battery powered, pocket sized VNA's. You don't need to worry about the influence of the test gear adding stray circuit paths.

But you do need to make the input Z and gain measurements separately, to avoid unbalancing the input during the Z measurements. If the output is connected to VNA port 2, it will ground one of the amplifier inputs.

As an experiment, I just tried adding a 1:1 transformer on the input, and it made practically no difference to the measurements.

Regards,

Martin

Ok, I stand corrected, as I was doing things wrong, both maths and sweep:

1-The VNA was being saturated with 10dB above its dynamic range.
2-THe mismatch attenuation calculations were wrong, so the input level to LZ1AQ is actually around -20dBm

After inserting a 25dB attenuator on the output of LZ1AQ, I got the following sweep:

/g/loopantennas/photo/296346/3804920

So, Steve, you were right. There is some evident gain oll off, but still, those long capacitor leads are worsening it. It might be worth shortening the input path.

The corrected gain values, adding the mismatch attenuation factor is as follows:

1MHz
S21=25,855dB
Mismatch attenuation = 7,051dB
Gain = 32.906dB (S21 + Mismatch attenuation)

?

3.61MHz
S21 = 25,867dB
Mismatch attenuation = 6.973dB
Gain = 32.84dB

7.09MHz
S21 = 25.607dB
Mismatch attenuation = 6.934dB
Gain = 32.541dB

14.34MHz
S21= 24.468dB
Mismatch Attenuation = 6.604dB
Gain = 31.072

30MHz
S21=21.345dB
Mismatch Attenuation = 4.14
Gain = 25.485dB

I am not happy with the -20dbm at LZ1AQ input, as this leads to almost 13 dbm output at 1MHZ which might be reaching the compression region. I might try inserting an attenuator on the input and compare results.

73

Recently, I have assembled several LZ1AQ preamps on FR4 1.5mm 2-sided PCB. In the album below, you can find the photos of the assembled preamps, graph of measured and modeled gain, photo of dummy aerial (aerial simulation network) and circuit of dummy aerial.
/g/loopantennas/album?id=296366
In case the link above doesn't work, look for the album "9W2LC's LZ1AQ amp". I have a problem with the simulation not matching the measurement. So, I am looking for any help / suggestion. This is still a work-in-progress because I haven't measured the P1dB and OIP3.

Digression: Not active loop, but I am searching for measurement of the PA0DRT & ON1BES miniwhip for the purpose of validating my simulation. I will be grateful if someone can point to where the data is available. Thanks.
73,
Chin-leong Lim, 9W2LC.9
.
W2LC's LZ1AQ amp

On Sat, Jul 13, 2024 at 03:43 PM, <biastee@...> wrote:

photo of dummy aerial (aerial simulation network) and circuit of dummy aerial.

Hi Chin-Leong,

That's interesting, was that the circuit you came up with after our previous discussions about how best to simulate a loop in the absence of a standard method ?

I have uploaded my original simple simulator circuit, for others to better understand the context of this discussion.

/g/loopantennas/photo/267767/3805080

/g/loopantennas/photo/267767/3805081

Regards,

Martin

Hi Martin,

Also I noticed that schematic diagram of Biastee's having presented the loop antenna as a dummy load in order to run S21 test with VNA.
In my opinion, the layout should be changed differently:
1. Yes, between CH0 port of VNA shall be a fixed attenuator, say for 20 - 60 dB. However, it shall be Pi configured for balanced connection with a shunt arm - active resistor of value 2 ohms ( that resistor would represent the loss of antenna ).
--- |--------^^^^^---|------~~~~~~~~~~~~~~~~|----------->
R1=50 ohm R3 R2=2 ohm L1=700 nH = Ca<50 pF to amp under test
|--------^^^^^---|------~~~~~~~~~~~~~~~|------------>
GND R3=R4=10 ohm - 3Kohm L2=700 nH

2. Connect L1=L2=700 nH as series arms and an additional shunt arm with capacitor Ca<50 pF
3. The LPF might be connected as well as based on actual situation FMBS interferers.

Regards,
Raphael

On Sat, Jul 13, 2024 at 06:04 PM, Raphael Wasserman wrote:

LPF might be connected as well as based on actual situation FMBS interferers.

Hi Raphael,

I quickly tried Biastee's circuit, but it didn't work out too well for me.

I still favour my version, which gives results that are close to an actual loop, but perhaps still not close enough.

/g/loopantennas/message/19705

Regards,

Martin

开云体育

For my loop preamp measurements, I use a jig between the VNA and the preamp, shown here:

- unbalanced to balanced transformer is now different, on T50-6 core, quadrifillar wound. Primary is 10+10T (center connection floating), secondary is 10T and 10T
- jumpers to change the output between in-phase (for common mode suppression measurement) and differential.
- Jumper outputs are connected to 24Ohm to 0.5Ohm dividers, serving as an attenuator. It mimics the loop impedance (somewhat, at least on the lower frequencies). This also results in a good S11 on the VNA side.

No additional attenuator is used. LZ1AQ gives about +5dBm at the peak of the curve, no problem for NanoVNA.

73, Mike AF7KR

Yes.