John, You really need to use accurate test equipment, preferably a service monitor, to verify your frequencies. Unless your repeater and duplexer are exactly on frequency, your desense problem could possibly be due to the notches on the duplexer not being coincident with the TX and RX frequencies of the repeater.
The Yaesu FTR-1510, like the similar low-tier products from Kenwood, Motorola, Icom, and Ritron, is intended for less-stringent applications- construction sites, shopping malls, race tracks, etc. As I have mentioned in other postings to this list, some of these boxes are equipped with single-braid coaxial cable to connect various modules within the enclosure. The Motorola GR1225 and Vertex VXR-5000 are examples of this cost-cutting practice. I have corrected significant desense in these two models by replacing the internal jumpers with RG-400/U cable. One way to settle this issue is to first program the FTR-1510 as a base station, or simply disable the repeat function, and program your TX and RX frequencies without any CTCSS tones. Put a suitable dummy load directly on the repeater's TX output connector, preferably one that screws onto the output connector itself, without any cable or adapter fitting. Using a service monitor, inject a signal at the RX frequency directly into the repeater's receive connector. The test signal should be frequency-modulated with a 1 kHz tone set for 3 kHz deviation. Monitor the de-emphasized receive audio using the SINAD meter function, and set the amplitude to achieve a 12 dB SINAD reading. If the repeater is properly tuned, this signal level will be in the range of -119 dBm to -113 dBm, or 0.25 to 0.5 uV. Now, while watching the SINAD meter reading, key the transmitter. If the SINAD meter reading drops significantly when the transmitter is keyed, you have in-cabinet desense! The cure is probably going to require making up new double-shielded cables. I don't know what's inside the Yaesu FTR-1510, but I will assume that three cables are needed: One cable from the bulkhead RX jack to the receiver input, one cable from the exciter output to the PA input, and one cable from the PA output to the bulkhead TX jack. Before replacing any cables, check for loose or missing shields, missing cover screws, loose connectors, etc.- anything that might allow RF to sneak in where it doesn't belong. If the above test proves that the desense is not caused by leakage within the repeater enclosure, check to see if the duplexer frequencies exactly match those in the repeater. It is important to use the same equipment to check the duplexer and the repeater; it is more important for the tuning to be the same rather than be exactly on the center frequency. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Sunday, August 31, 2008 8:42 AM To: [email protected] Cc: [EMAIL PROTECTED] Subject: [Repeater-Builder] Wits End -- Desense I've tried everything, it seems, and I still have desense!! Even when I connect only the repeater (Yaesu Musen FTR-1510) and a controller (needed to make the repeater transmit) and put a dummy load on the TX out, I get desense. Following up on Eric's suggestion about holes leaking RF, I sealed the edges and holes in the TX and RX units inside the repeater, and I built a shield to enclose the back side of the TX connector that is on the back side of the repeater. That might have reduced the desense a little bit, but not much. I even ran the TX feedline to an outside wire-mesh chair in an effort to reduce any possible radiation getting from the dummy load to the receiver. There was still the same desense. The desense is at least 10 dB. It appears to me that the desense has to be occurring inside the repeater cabinet, but for the life of me I can't see how this can be. Everything seems to be well shielded. So, I'd very much like to hear your theories and suggestions. Is there some way to find the source of the desense radiation? Is there some way that unshielded control lines, audio lines, and power lines can carry RF to the receiver? I have looked at the output of the repeater TX with a spectrum analyzer [tnx Tom N4ZPT] and it is clean. While I don't think the following is significant, for completeness I note that the repeater RX and TX both appear to be several kilohertz low in frequency. However, I do not have a frequency counter, and I am only checking the frequencies by the use of an HT and mobile, both modern transceivers by Yaesu. Your thoughts, suggestions, and sympathy will be appreciated. John AF4PD
