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Post date: Feb 11, 2021 8:55:19 AM
Brewer Pedin W4MGA performed some updates to the KG4MRA repeater. The DR-2X was set to 20 WPM for the Morse ID (reduced from 32 WPM), the operational mode was changed from Full Fusion C4FM (uplink and downlink) to a split mode of AMS (C4FM or NBFM) on the uplink, and NBFM only on the downlink. Users are welcome to continue to use C4FM on the uplink, and their transmissions will be de-vocoded and retransmitted as NBFM on the downlink. Users that access the repeater with C4FM will sound like vocoded speech on the repeater output, even though it is transmitted as NBFM. NBFM on the uplink also is retransmitted as NBFM per usual. The CTCSS remains 74.4 Hz for both uplink and downlink.
Also, the Henry VHF Power Amplifier was reinstalled (after repair) and plumbed into the system. TX output was significantly increased (200+ Watts), and measured by remote stations out past the 50 mile radius, yielding a +6 dB from the no amplifier 50 Watt max output of the DR-2X. Unfortunately, this caused some repeater de-sense (reduced sensitivity due to broadband noise on the repeater RX channel), and so the PA was shutdown, until the de-sense problem can be resolved. The newly installed (at Henry Radio) RF bypass relay now allows the DR-2X TX output to pass through the unpowered Henry C250AB10R.
The de-sense could be the lack of grounding and shielding, or other isolation issues. The grounding in the system rack definitely needs some attention, as does the row of racks in the TX building. But more likely the de-sense results from the fact that, a DR-2X is not much more than a pair of Yaesu FTM-400 Xcvr bricks in a rack mount box, with a host controller, display, and power supply. The DR-2X is designed to cover the entire 2m and 70cm bands, settable by front panel, so the TX/RX are broadband designs, settable frequency by front panel (like the FTM-400 bricks are themselves), not like the commercial Motorola repeaters, which are designed to be optimally tuned via high Q RX preselectors, like 5 stage cascade helical tuned resonators. The old Motorola Quantars have the 5 stage cascade of helical resonators as part of the RX module, and those are hand tuned on a bench, as part of the commissioning prior to installation. Further testing, modifications, grounding, preselection filters on the RX imput of the DR-2X will probably solve the de-sense, and ultimately if those measures don't work, reducing the output power of the PA to whatever level required, is an option. The 6 can Bird TX/RX Systems duplexer of KG4MRA has very high/good isolation between the RX and TX frequencies, but the isolation is not infinite, and at some level of TX power, the broadband noise of the high power PA will creep up to the threshold of the repeater RX, and interfere with low level (at threshold) Amateur Radio users, which was the case currently. Look for updates here in Announcements Tab for future effort and results.
73's
Brewer W4MGA
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