RE: [Repeater-Builder] Re: desense question
Just watch the power dissipation. You wouldn't want to risk melting the coax. There's next to no cooling - conduction, convection or radiant from a sealed paint can. My rule of thumb - figure what the attenuator is going to dissipate - let's say that you are going to take a 20 watt transmitter and knock it down 2 watts. That's 18 w of dissipation. Double the number to account for some safety factor (makes 36w), then round the number up to the next larger light bulb (60w in this case). Mount a ceramic socket to the underside of the lid using a screw into a hole you can re-use later as one of the coax connector mounting holes. Use a coax connector center hole for the18ga wires to the light bulb socket then stuff some steel wool into the hole to plug it. Use a variac to set the bulb to dissipate 36w. Run it overnight. Once it stabilizes can you place the palm of your hand flat onto the side of the can and count to 20? If not, it's too hot and you need to go to a larger can (with more surface area). Maybe a 2.5 gallon can (rare, but they exist), or a 5 gallon can. A coat of flat black paint will help radiate the heat. Once your attenuator configuration passes the palm test, remove the lamp socket and the wiring, then mount the coax connectors and the coax. Mike At 06:31 PM 05/04/09, you wrote: Thanks Mike. I'll file that trick away for future use! Mike WM4B -- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Monday, May 04, 2009 7:10 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Re: desense question At 02:22 PM 05/04/09, you wrote: Mike, I assume the purpose of the paint can is to act as a Faraday cage? Yep. Cheap coax is lossy and leaky. Is it attached to the common-point ground system or left freestanding? If I remember the situation (it's been over 15 years since I shot the photo) it was freestanding (but the DC continuity went from the shield of coax #1 through the connector to the paint can lid to the connector #2). Of course it also went through the coax braid. The can was just in the coax line from the exciter to the PA deck. Nothing fancy, just two superflex jumpers and the attenuator can. 73, Mike WM4B From: Repeater-Builder@yahoogroups.com [ mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Monday, May 04, 2009 5:19 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: desense question At 06:07 AM 05/04/09, you wrote: The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI Somewhere I've got a couple of photos of attenuators made from a paint can with coil of coax inside it. A pair of hooded SO239s on the lid and you're done. One is of a 5 gallon paint can, the other of a 1 gallon. Mike
[Repeater-Builder] Re: desense question
The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI
RE: [Repeater-Builder] Re: desense question
I think the issue was that it's RG-58. Mike WM4B On Mon, May 4, 2009 at 10:15 AM , Al Wolfe wrote: The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI http://groups.yahoo.com/start;_ylc=X3oDMTJuYW12NWlxBF9TAzk3MzU5NzE0BF9wAzMEZ3JwSWQDMTA0MTY4BGdycHNwSWQDMTcwNTA2MzEwOARzZWMDbmNtb2QEc2xrA2dyb3VwczIEc3RpbWUDMTI0MTQ0NTUxNQ--
Re: [Repeater-Builder] Re: desense question
At 06:07 AM 05/04/09, you wrote: The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI Somewhere I've got a couple of photos of attenuators made from a paint can with coil of coax inside it. A pair of hooded SO239s on the lid and you're done. One is of a 5 gallon paint can, the other of a 1 gallon. Mike
RE: [Repeater-Builder] Re: desense question
Mike, I assume the purpose of the paint can is to act as a Faraday cage? Is it attached to the common-point ground system or left freestanding? 73, Mike WM4B From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Monday, May 04, 2009 5:19 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: desense question At 06:07 AM 05/04/09, you wrote: The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI Somewhere I've got a couple of photos of attenuators made from a paint can with coil of coax inside it. A pair of hooded SO239s on the lid and you're done. One is of a 5 gallon paint can, the other of a 1 gallon. Mike image001.jpgimage002.jpg
RE: [Repeater-Builder] Re: desense question
Thanks Mike. I'll file that trick away for future use! Mike WM4B _ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Monday, May 04, 2009 7:10 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Re: desense question At 02:22 PM 05/04/09, you wrote: Mike, I assume the purpose of the paint can is to act as a Faraday cage? Yep. Cheap coax is lossy and leaky. Is it attached to the common-point ground system or left freestanding? If I remember the situation (it's been over 15 years since I shot the photo) it was freestanding (but the DC continuity went from the shield of coax #1 through the connector to the paint can lid to the connector #2). Of course it also went through the coax braid. The can was just in the coax line from the exciter to the PA deck. Nothing fancy, just two superflex jumpers and the attenuator can. 73, Mike WM4B From: Repeater-Builder@yahoogroups.com [ mailto:Repeater-Builder@yahoogroups.com mailto:repeater-buil...@yahoogroups.com] On Behalf Of Mike Morris WA6ILQ Sent: Monday, May 04, 2009 5:19 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: desense question At 06:07 AM 05/04/09, you wrote: The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. Not a thing wrong with using a length of coax as an attenuator if it's done right. Have used this technique often when a little attenuation is needed. Al, K9SI Somewhere I've got a couple of photos of attenuators made from a paint can with coil of coax inside it. A pair of hooded SO239s on the lid and you're done. One is of a 5 gallon paint can, the other of a 1 gallon. Mike
Re: [Repeater-Builder] Re: desense question
Skipp. I will post a pic with the filtering specs later today. Its all here at my qth. Ian. Va2ir. Sent on the TELUS Mobility network with BlackBerry -Original Message- From: skipp025 skipp...@yahoo.com Date: Sun, 03 May 2009 03:50:00 To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: desense question va...@... wrote: Running the hamtronics REP-200 with the optional 15 watt PA in it. After all the filtering, I get a whopping 8 watts out. Is the filtering you mention on the 15 watt PA board or is it external to the repeater chassis (on the outside of the box)? 8 watts is a very usable value for driving an external amplifier if you have the proper duplexer or antenna system in place. Lack of duplexer isolation and filtering will quickly come back as a problem generator. Many of the Hamtronics Receiver models are fairly hot front end wise. I put a small strip amp inline with the TX port of the repeater, before the filtering, and it caused desense. Maybe tossing spurs - I have no way to test. If I understand what you're trying to describe, what you tried is probably not a good thing. Location is also not great for the moment, and the antenna is very temporary. A Diamond x500 connected with COAX (please don't shoot me). The club antenna will be down off the old tower (8 bay Sinclair) and I do have the heliax for it. Nothing wrong with coax when you understand what occurs at UHF, which is most/much of the signal is lost (both transmit and receive) in long sections. Pretty much any non high quality and type coax is an unforgiving signal resistor (loss). Unless you're using a decent type double shielded coax... I would suspect most coax types also make a surprisingly good antenna (very leaky - both tx rx directions). The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. I'm doing the same thing as part of the 224 MHz home brew repeater project I started describing in a post made earlier today. Pictures of in the group photos section. http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list Is this my problem? Lack of shielding causing desense? When I put everything back to normal, my test station was solid copy. Hard to say without knowing more about your duplexer or cavity filter components. If you bump the Tx Power up, you must also ensure you have enough receiver protection in place. What is my best option to get a little more oomph on the output with out tossing megabucks at it? Thanks Ian VA2IR The budget minded repeater owner/builder would probably put a mobile amplifier on the repeater. Better to not get greedy like many do and shoot your project in the foot. On your repeater I would expect 25 to 45 watts to be a very reasonable value. Keep in mind you'd better have a decent duplexer or antenna system in place or you'd better upgrade what you have. In many but not all cases a small blower (not a fan) moving air across the amplifier heat sink after modest tx talk time should be considered. Many repeater controllers have fan/blower control considerations built into their software and hardware logic. Many stuck up repeater builder types will tell you that using a mobile amplifier in a repeater application is a horrible idea. I can give you many examples and reasons where and why it's not the big sin many hard nose profess... but let's save that topic for another day. If you're properly dealing with the heat sink heat with proper air movement and/or duty cycle management, then by all means get on with other more important remaining issues. Sometimes you've got to work with what you have and when properly integrated into a system, you can use a mobile RF Amplifier in a repeater situation. cheers, s.
[Repeater-Builder] Re: desense question
va...@... wrote: Running the hamtronics REP-200 with the optional 15 watt PA in it. After all the filtering, I get a whopping 8 watts out. Is the filtering you mention on the 15 watt PA board or is it external to the repeater chassis (on the outside of the box)? 8 watts is a very usable value for driving an external amplifier if you have the proper duplexer or antenna system in place. Lack of duplexer isolation and filtering will quickly come back as a problem generator. Many of the Hamtronics Receiver models are fairly hot front end wise. I put a small strip amp inline with the TX port of the repeater, before the filtering, and it caused desense. Maybe tossing spurs - I have no way to test. If I understand what you're trying to describe, what you tried is probably not a good thing. Location is also not great for the moment, and the antenna is very temporary. A Diamond x500 connected with COAX (please don't shoot me). The club antenna will be down off the old tower (8 bay Sinclair) and I do have the heliax for it. Nothing wrong with coax when you understand what occurs at UHF, which is most/much of the signal is lost (both transmit and receive) in long sections. Pretty much any non high quality and type coax is an unforgiving signal resistor (loss). Unless you're using a decent type double shielded coax... I would suspect most coax types also make a surprisingly good antenna (very leaky - both tx rx directions). The amp is a UHF PA off a mobile rig, and I needed about 50 feet of RG58U to attenuate the signal from the repeater into the amp module. Not good, probably better to bypass (not use) the 15 watt amplifier and drive the external amp direct from the exciter. I'm doing the same thing as part of the 224 MHz home brew repeater project I started describing in a post made earlier today. Pictures of in the group photos section. http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list Is this my problem? Lack of shielding causing desense? When I put everything back to normal, my test station was solid copy. Hard to say without knowing more about your duplexer or cavity filter components. If you bump the Tx Power up, you must also ensure you have enough receiver protection in place. What is my best option to get a little more oomph on the output with out tossing megabucks at it? Thanks Ian VA2IR The budget minded repeater owner/builder would probably put a mobile amplifier on the repeater. Better to not get greedy like many do and shoot your project in the foot. On your repeater I would expect 25 to 45 watts to be a very reasonable value. Keep in mind you'd better have a decent duplexer or antenna system in place or you'd better upgrade what you have. In many but not all cases a small blower (not a fan) moving air across the amplifier heat sink after modest tx talk time should be considered. Many repeater controllers have fan/blower control considerations built into their software and hardware logic. Many stuck up repeater builder types will tell you that using a mobile amplifier in a repeater application is a horrible idea. I can give you many examples and reasons where and why it's not the big sin many hard nose profess... but let's save that topic for another day. If you're properly dealing with the heat sink heat with proper air movement and/or duty cycle management, then by all means get on with other more important remaining issues. Sometimes you've got to work with what you have and when properly integrated into a system, you can use a mobile RF Amplifier in a repeater situation. cheers, s.
