I purchased this 70 Watt Amplifier Kit and built it. Unfortunately, the kits do not come with any instructions. I have managed to find a variety of web sites where people built the amp and offered perspective. The kit seems to be shipped in varying configurations. In any case, I built it and it does not work properly. It basically amplifies, but the input SWR as seen by my Ham radio is high. I am using a 50 Ohm dummy load as output. I have decided to model the device in LTSPice to see what is going on and work on the source impedance problem.

Here is a link to a site describing the amp/more info


The pictures show the transformer labelled as having a 2T:0.5T turns ratio. When the amplifier is active, the input signal goes thru an inductor(the transformer) to ground. My radio sees a high SWR (at 3.9 MHZ). The resister labelled R6 (18 Ohms)/Vertically placed was not described on some web sites, but another said to include it. The resister is over the secondary side of the transformer.

When I wound this transformer, I only used one piece of wire, as per most of the web site descriptions. I believe what I have shown in the photos is a binocular transformer? If it is not, please correct me. I am not sure how it can be built with only 1 wire, but I did it. How is the secondary linked?

One side of the transformer is soldered to a single contact area. The other side is soldered to 2 contact areas. The resister is over these two contact areas.

I wish to model this transformer in LTSpice. In LTSpice I can set up 3 inductors and use a K statement to link them. I am wondering how to determine the proper inductances to use for my LTSPice model. I understand the turns ratio of 2:0.5 would govern the ratio of the inductances on the primary and secondary. Aside from the ratio, how do I determine the inductance to use for the primary side (in Henries)? I have an inductance meter (LCR). Could I unsolder the two yellow wires , shown in one of the photos , and measure the inductance? If I did this, would the reading be muddied because the transformer is soldered to the board and the secondary is connected (somehow)?

If I use the turns ratio to determine an inductance to use for the secondary side of the transformer, is that value to be divided by 2 and then used for each of the two transformers on that side. The secondary side looks like it is center tapped.

Please post some advice on this modelling problem. I think my first uncertainty is the inductance value to use for the primary side?

If anyone knows what might needed to reduce the SWR as seen by my radio, that would be helpful. One person mentioned a cap in series with the transformer input. At first, I'd like to see this modelled in LTSPice and then test things there.


Side View

Top View

Circuit Diagram

  • \$\begingroup\$ "binocular transformer" is an amusing word for it, but fitting! \$\endgroup\$
    – Hearth
    Jul 26, 2021 at 20:06
  • 1
    \$\begingroup\$ LTspice Using Transformers \$\endgroup\$ Jul 26, 2021 at 20:36
  • \$\begingroup\$ How does the SWR change over the whole frequency range? \$\endgroup\$ Jul 26, 2021 at 21:16
  • \$\begingroup\$ Have you tried it without the 18Ω resistor? \$\endgroup\$ Jul 26, 2021 at 23:07
  • \$\begingroup\$ I have not tried it without the resister. I will test that. I found a table that listed various resisters and SWR on the different bands. The sizes were 18 - 21 ohms. \$\endgroup\$ Jul 26, 2021 at 23:18

1 Answer 1


T1 is a 2 hole oval ferrite slug with a 2T:0.5T turns or 4:1 turns ratio “0.5T “ is a U shape loop

T2 is a dual cylindrical ferrite slug that seems to be mounted on FR4 at both ends with 0.5T:3T turns.

All the turns must be done with Litz wire a dozen or two doz. fine magnet wires in parallel to significantly reduce the skin effect losses.

Use the finest Litz wire gauge you can get your hands on that fits in the area avail.

e.g. AWG 40 to 48 strands min.

Skin Effect depth at 30MHz = 12 um.

AWG40 radius = 40um !

enter image description here

The FR4 might have mu foil to reduce end flux leakage on T2.

  • \$\begingroup\$ Barev, Any questions or doubts? \$\endgroup\$ Jul 27, 2021 at 2:07
  • \$\begingroup\$ What do you mean “0.5T “ is a U shape loop? I think you mean that the middle of the U shape defines the .5 Turn side in the schmadic? It seems the number .5 is arbitrary as long as the proper ratio is achieved. Or, does the .5 just convey that it is center tapped ... but still can be modelled as separate transformers? I am still experimenting with this amplifier. I now see that using the MRF9120 is a better option than using the IRF 5130 mosfets. To get more power... I get 40 watts from 2 watts... I am going to use Litz wire and compare. \$\endgroup\$ Sep 13, 2022 at 14:36

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