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4m Update

April 29th, 2014 | Posted by Gavin, M1BXF in General

So 4m has not had the best start.  We are experiencing higher than usual inter-station breakthrough from HF into the 4m RX, here is an example of it at its worst;

4m_QRM_270414

This is a combination of about 3 HF stations, and as you can well see it’s impossible to work anything on 4m through this QRM.

Lots of investigations ensued and found that even at low power levels, 100w, and with all stations using band-pass filters, the interference was still apparent and we had no way to cure it, the only conclusion is we have something nearby which is causing a “rusty bolt effect”, there is little chance of curing it.

The site we are operating from is an old Decca transmitter site used for navigation and we believe there is a network of ground radials underfoot and the building has an integrated faraday screen, and probably many other pieces of metalwork we have yet to see.

The good news is we have found a combination of bands which work now well with 4m and 4m operation is yielding QSOs again, so if you see us on the band, or on ON4KST chat, then give us a call.

One issue though, and it’s a biggie, is during the investigation phase, alignment of a backup transverter was in progress when it went into TX oscillation which resulted in too much RF power being delivered into the MRF-151G final PA which in turn destroyed the device.  The workaround has been to use the Yaesu Quadra on 4m as the final PA driven from the G4DDK 7w RA07H0608M pre-drive PA, this setup can achive 80w out the Quadra on 4m, this isn’t so bad as it’s only 3dB down on the MRF-151G output.  It does mean however 6m must run barefoot (80w) from the K3 when 4m is using the Quadra but can be set back to 400w when 4m is not in use.

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