mcHF, a build it yourself Amateur Radio HF rig (80-10M bands)

Tx mods.

The mcHF was developed by Chris, , and is a fully self contained SDR rig with a colour display. Details are available here.............. PCB's and kits are available to build this project............ OR, buy blank PCB's and built it yourself. It's highly recommended to join the mcHF Yahoo Group here.

Current versions of the PCB's is 6. VK3PE has built a version using V0.4 PCB's. The original design used a 3D printer made case. Some pre-made metal cases are also available but ny case was made using standard aluminium extrusions and probably takes longer than the electronic assembly side of the project. A milling machine though was available for some of this work.


Assembly went fairly easily and quickly, using just a magnifying desk lamp, tweezers, (project is SMD) and the usual soldering iron and other hand tools.

There are a number of reccomended 'mods' for this rig, as there always are in any developing project. Several Amateurs have come up with various changes, but I used the information here to modify the PA stages mostly, due to low power on 10M.

One problem I found when testing, was the presence of 'spurs' in the Tx spectrum output, close to the carrier. No amount of balancing (I&Q phase, levels) could get the spurs to confirm with Australian authorities spec of -46dB. Could get close but not comfortably exceed this required Spec.

The on-line Group discussed this over some period but no real solution was found although the consensus seemed to be, the Tx Mixer is being overloaded. One way around this is to remove the gain stages prior to the Mixer and replace the missing gain at a point after the mixer. Chris has done some tests on this in late 2016 fitting a 10dB gain stage after the Mixer, having removed the LM386 gain stages.

In April 2017, VK3PE decided to look at this also and do some measurements. (Some of the results 'before' were tested in 2016 before the Mods were made, for comparison purposes.) I used a Mini-Circuits™ MMIC, type GVA-81+ for my version, with approx. 12dB gain, a little more than the 10dB Chris suggests.

Unfortunately, I could not duplicate Chris's results after the changes. (see Chris's result in link in the previous paragraph) [The "tune" button was used in all tests and translate offset used.]

STOP PRESS !!......There have been some further changes and test made by others in the mcHF Group. I am currently looking at changing the 74AC74 to a Potato chip (74G74) when it arrives, plus reducing the suppy voltage to this chip to minimise LO feed through of the Mixer. More tests will be done then.

Plots of output before mods. were made:-


Chris's spectrum plot.


VK3PE plot.


Plots after changes were made:-

fit Potato chip


Change Potato chip supply from 5v to 3v3



Page uploaded on April 20th, 2017

Last updated on May 31, 2017