U3S output ( single band unit versus LPF board unit )

Vernon Matheson

Hi all,

I have built several U3S and 5 watt PA units over the last few few years and one thing that I have noticed especially when using 10 meters is the single band unit with the LPF in the zero slot has more output than when in the #1 slot on the LPF board unit.

This really doesn't make a lot of sense to me as it is with the same antenna, PSU, etc.

I have tried changing from one amp to the other and still get the same results.

With the single band unit I can achieve 5 watts on 10m and run the amp at 12 vdc and it hardly gets luke warm and with the unit and LPF board I can get a max of 2 watts.

Anyway just wondering if anyone else has experienced this.


Cathal Ferris

Hi Vernon,

Just to clarify.

You have one U3S kit, built with the LPF relay board, and with the PA unit inline between the U3S and the LPF, and another U3s kit built as a single band but also using a PA?
That you see an output power difference between the two kits, multiband kit with the 10m filter in slot 1 and the single band kit?

Or are you comparing the same U2s kit with and without the LPF in place? If so, are you correctly putting the PA between the U3S and the LPF relay board? If not, do you have the appropriate LPF between the PA and the antenna?

Are you using the same physical 10m LPF filter to do the comparison?
Is there a difference in the power draw by the U3S between the two configurations?

If it's a comparison between a single band U3s and a multiband U3s, it could entirely be down to construction and component differences between the kits. 

Always fun trying to troubleshoot things like this, but a better understanding of the current setup is needed.


Vernon Matheson

Hi Cathal...

I am comparing a sigle band U3S with 5 watt PA kit and a multiband U3S with a 5 watt PA kit

Doing the comparison i used the same 10M LPF  as well as the same 5 watt PA kit for testing each setup.

In the single band the LPF is in slot zero and in the multiband in slot 1 as that is where the LPF with the highest frequency is supposed to be installed on the LPF board.

Each U3S uses the 25 turn T1

The 5 watt PA is between the U3S and LPF in both cases.

It's not a big deal but just made me wonder why.

I haven't checked current draw yet.



Hi Vernon,
have you tried measuring the power on the one fitted in position 1 with all other lpfs removed? I have had an issue with two faulty relay switched lpf boards where more than one lpf is in circuit, effectively two lpfs in parallel, and this gives a low power output. 

73 Ken G4APB.