RE: [Repeater-Builder] Trying to test my antenna system (suspect desense..)
Doug, Please advise the make, model, and power of your transmitter, and the make and model of your receiver and duplexer. That information will help the Elmers on this list arrive more quickly at possible causes. It will also be helpful to state the make and model of your antenna, and the type and length of the feedline. What is the transmitter power output (before the duplexer) into a dummy load, and what is the 12dB Sinad sensitivity of the receiver? I hope that the jumpers are all double-shielded cable, such as RG-400 or RG-214; are they? The MFJ antenna analyzer is really useful only to measure the VSWR of the antenna, by itself. Correct tuning of a duplexer is best done with a network analyzer, measuring return loss. 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com [mailto:Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com ] On Behalf Of Doug Hutchison Sent: Friday, June 12, 2009 8:26 AM To: Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com Subject: [Repeater-Builder] Trying to test my antenna system (suspect desense..) Suffering from poor receive perfomance and am trying to check integrity of system. Single 3 dB antenna, single output 6 cavity filter, 10w out of filter. Could someone enlighten me, the cavities are factory tuned 145.6875 tx 145.0875 rx and am attempting to check VSWR with an MFJ antenna analyser. The answers I see do not conform to my understanding so, at what frequencies, at the input and output of the filter while connected to antenna, should I see low vswr. I'm seeing 1.2:1 or so at 146.08, should I not see that on 145.08? Just a bit confused! Thank you, Doug - MM0BJA
RE: [Repeater-Builder] Trying to test my antenna system (suspect desense..)
Doug, If you are measuring the VSWR at the TX Out you will see the VSWR of the cavity and the antenna. If the cavity is not properly tuned you may see a high VSWR. To check this put a dummy load on the antenna port of the cavities. If the VSWR is less than 1.5:1 then you are good. If it is higher then the cavities may need tuning. You can also check the RX side of the cavities the same way with a mobile transmitter. Put the TX on the receive frequency. If the VSWR is high the cavities need to be tuned. Checking the antenna is the easiest. Connect the transmitter to the antenna and see what you get. Remember the antenna may not be tuned to the lower part of the band, but should be in spec. If the antenna has a high VSWR is the lower part of the band, it can have an adverse effect on the cavity performance. You can also get a good feel of the tuning of the cavities by checking the power out of the transmitter and then the tx power out of the antenna port. Typical loss is between 1.5 dB to 3 dB depending on the number of cavities in each side. If you have 100 watts out of a transmitter into a dummy load or going into the cavities and get between 40 and 60 watts out I would say the cavities are probable working. The same can be done on the receive side. It's not hard, but to the untrained or unskilled it seems hard. Check your manufactures tuning specs that you received with the cavities. They will tell you what dB loss you should have, then go from there. Good Luck. Charles Miller, WD5EEH -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Doug Hutchison Sent: Friday, June 12, 2009 8:26 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Trying to test my antenna system (suspect desense..) Suffering from poor receive perfomance and am trying to chech integrity of system. Single 3Db antenna, single output 6 cavity filter, 10w out of filter. Could someone enlighten me, the cavities are factory tuned 145.6875 tx 145.0875 rx and am attempting to check VSWR with an MFJ antenna analyser. The answers I see do not conform to my understanding so, at what frequencies, at the input and output of the filter while connected to antenna, should I see low vswr. I'm seeing 1.2:1 or so at 146.08, should I not see that on 145.08? Just a bit confused! Thank you, Doug - MM0BJA Yahoo! Groups Links
[Repeater-Builder] Trying to test my antenna system (suspect desense..)
Suffering from poor receive perfomance and am trying to chech integrity of system. Single 3Db antenna, single output 6 cavity filter, 10w out of filter. Could someone enlighten me, the cavities are factory tuned 145.6875 tx 145.0875 rx and am attempting to check VSWR with an MFJ antenna analyser. The answers I see do not conform to my understanding so, at what frequencies, at the input and output of the filter while connected to antenna, should I see low vswr. I'm seeing 1.2:1 or so at 146.08, should I not see that on 145.08? Just a bit confused! Thank you, Doug - MM0BJA
