144MHz

Our 144MHz equipment is marked by big homemade PAs and multiple antenna systems with antenna switches. The antenna strategy is varying from contest to contest.

Transceiver
We are currently running commercial HF/VHF - transceivers ICOM IC-746 ( See a ordinary setup below :)

This seams to be strange regarding the QRM situation on 144MHz and the intermodulation capabilities of the "modern" Rx.
But lets compare our setup with the well accepted "contest standard" LT2S + HF- transceiver and demonstrate some myths and facts!
The table below shows the overall IP3 of some usual HF - rigs + LT2S based on ARRL measurements of the 10m - IP3 and the LT2S datasheet :

HF RIG

LT2S

NF [dB]

IP3 [dBm]

FT-1000MP

3,2

-13

TS-870

3,2

+0

TS-850

2,7

-4

IC-775DSP

1,8

-18

IC-756PRO

2,1

-6

IC-738/736

2,2

+1

Table courtesy of S53WW

S53WW shows in his fine article about a high performance 144/14 transverter that the most popular HF - rigs on 10m have bad IP3s and so the combination with a LT2S gives only marginal results.
What are the capabilities of "our" IC-746 ? The ARRL measures an IP3 of -2.9 / -2.4dBm ( with preamp off/on ). This is quiet an acceptable result.
Yes, there are several ways to push the IP3 up to +4dBm. Use some kind of R&S reference receiver or choose another band for downlink as described in the article above. For us, the overhead is not suitable except in the September contest.

High performance transverter

The only contest witch legalizes an extraordinary setup is the September contest : In 2004 we tried the new TR-144H transverter by DB6NT : Because of the technical data given by a TS-950 no mathematical increase of IP3 was expected, but we found an impressive enhancement in reality which is estimated between 5-10db by the crew.

The second Rx is an IC-746 witch is cross connected to both antenna systems via antenna switch and the audio is switched with an audio switch box. The 2nd OP had the chance to switch between 1st Rx only, 2nd Rx only and "stereo" and could operate the 2nd Rx independently. So he could either support the first OP on our "home frequency" or search the band for new stations or look for the competitors or both.

Tetrode power on 144MHz
The PAs are homebrewed with Russian tubes by DK4RC who has a long experience in building high power amplifiers. We are running only tetrodes GU-74, GU-43 and GS-23 with low intermodulation distortion and high gain.
See the circuit principles below :

Schematic diagram tetrode PA

Construction top view

Construction side view

The designs can provide 750w output from only 20-30w input power. The low drive power needed by such tetrode PAs reduces the intermodulation of the VHF-Tx noticeably.

Antennas
The antenna setup varies from contest to contest. This depends on the strategy and on the personal resources.
The main antennas operated by "armstrong rotor" are 2 x 11el Flexa or 4 x 11el Flexa. The 2nd antennas are either 2 x 11el Flexa or 2 x M² 2M5WL as shown on the pictures.

Antenna switch
The antenna switches are used to split the output on two antenna systems in TX - mode and to select an antenna system quickly in Rx - mode. There are several operation modes and principally no differences between the bands. See 432MHz for details.