| Author |
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CM 3885
| | Posted on Friday, December 13, 2002 - 12:43 am: | 
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I know some of you might be like "HUH?" but i used to know an old timer that worked on CB radios up until he passed in 1996 and he had what he called a "5 mile dummy load"
Ok, what it amounted to was a home made dummy load that he used to test radios when there wasnt anyone on the air locally. For instance he wanted to check the sound of the modulation audio on say, a cobra 148 GTL. He used another rig like say a cobra 2000 base he then hooked the "5 mile dummy load" to the back on the 2000. He then turned up the volume of the 2000 while trasnmitting on the 148. What it did was make the 148 sound like someone was actually talking on the antenna and the 2000 was sitting at a base 5 miles away in another town! This device he had actually worked like a champ! No squeals or feeback. He said the 5 mile load was only good to check the receive of a radio but it was not meant for transmitting. My qusetions is does anyone know how to bild one of these? I never had a chance to get the one off of him whe he passed but i always wanted to have on of my own. |
Jp1116
| | Posted on Friday, December 13, 2002 - 8:15 am: |
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It was out of a Secret CB book. I have it at home but don't remember the number right off. (Maybe 15????)I can scan it and send it to you if you would like. |
ChillyDog
| | Posted on Friday, December 13, 2002 - 10:43 am: |
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CM 3885,
I can think of two possibilities here: a regular dummy load or an in-line attenuator.
On my bench I'll sometimes use a dummy load to evaluate transmit audio quality. There is enough leakage using a dummy load that a 4 watt transmitter will put an S2 or S3 signal into a nearby receiver sitting on the same bench. Just about right for many of my purposes. If you need further transmit signal reduction you can short the receiver input.
You could also make or buy a 20dB or 30dB attenuator. You would have to make sure the power handling capability of the attenuator is adequate to handle the transmit power. 20dB of attenuation is a power reduction of 100 times, 30dB gives 1000 times. Typically, 20dB is about 3 S units, 30dB is about 5. Hook the attenuator between the transmitter and receiver using coax jumpers.
In either case, use headphones on the receiver to avoid setting up a feedback loop and generating a squeal.
Best Regards,
Bob |
Marconi
| | Posted on Friday, December 13, 2002 - 11:39 am: |
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I don't think there was anything special about that article that made the simulation = 5 miles. It was just a little cheap homemade dummy load. Maybe it was suitable for transmitting maybe not, but if you get yourself a good modestly priced dummy load, you can do the same thing and have yourself a good load that you could transmit into for testing also.
If you just want to do this, you can probably transmit from the radio with the antenna and hear on the receiver radio with nothing in the antenna jack at all. If that does not work for you, attach a jumper (maybe 6' long or (>) to the back of the receiver radio and that should improve the response a bit.
In either case, do not transmit on the radio set up to be the receiver, you could damage the rig. |
307
| | Posted on Friday, December 13, 2002 - 11:57 am: |
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Selman Enterprizes in Texas (Secret CB) used to make the "20 mile" version of what you are talking about..It was called the "Bandit CB Tester". I was in fact an oscillator (say channel 21) with an output of a few microvolts that was adjustable so that you could "simulate" listening to a radio 20 miles away...Cool little thing..Today you can do the same thing with a B&K 2040 CB Signal Generator...
307 |
Jp1116
| | Posted on Friday, December 13, 2002 - 5:54 pm: |
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It ended up being #5 and was called the five mile simulator. I'm sure there was nothing magical about the five miles, probably just a guess as to what the drop in signal strength was on the receiving unit. You could change the value of the resistors to get the effect you want. Using a real dummy load and monitoring on another receiver is probably the best way to go and the dummyload would be useful for other uses. The signal generator is great(and expensive)for working on receivers but it won't allow you to monitor the transmitted signal you want to hear. If you still want the info, email me and I will send it. |
CM 3885
| | Posted on Saturday, December 14, 2002 - 1:08 am: |
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Hey JP send me the docs so i can make me one of those cool 5 mile dummy loads.. |
Jp1116
| | Posted on Saturday, December 14, 2002 - 5:48 pm: |
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Email me at jp1116@netzero.net and I will sent it to you. If you did already I think it got deleted. |
bruce
| | Posted on Sunday, December 15, 2002 - 8:10 am: |
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I looked at your drawing it is exactly what i figured and should work. NOW it should be built in a SHELDED box to obtain the drop you need in the ARRL hanbooks atenuators can be found and will do exactly what your tring to do with this one. One more comment it will not prevent audio feedback if you have enough mike gain it will happen.
Bruce |
CM 3885
| | Posted on Monday, December 16, 2002 - 12:24 am: |
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JP1116
got the docs but i cannot print it because it is too big! LOL |
bruce
| | Posted on Monday, December 16, 2002 - 7:20 am: |
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I built a 2 meter one to test my low noise preampa it was -120 db and was built in several bud boxes fed with double shield coax. STILL i had to place one radio in a sheilded box with bypassing to get THAT much atuation. some on the amps had .4db noise figures and would with a good radio behind them hear signals down close to .1 UV at 144.200. If anyone builds one of these remember to use CARBON resistors even though they are noisey at rf and SHEILD the input from the output and always use GOOD coax a MUST! |
Jp1116
| | Posted on Monday, December 16, 2002 - 7:48 am: |
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Try using Microsoft Photo Editor and you can downsize them. I suppose that would be true for most editing programs, that's just the one I use. If you want one by snail mail, send an email to the address above and I will send it out. I found an old 2 pill amp that I'm going to gut and use to put mine in. Ready made case and SO-239's already installed, how lazy can you get? |
CM 3885
| | Posted on Monday, December 16, 2002 - 4:46 pm: |
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yeah ill give that a try there JP and let you know AOK! |
Jp1116
| | Posted on Tuesday, December 17, 2002 - 10:35 pm: |
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Well I threw one of these together real quick today. I had to leave out the 1.5meg ohm resistor to keep the signal strength down. So I basically have two 8 watt dummy loads that happen to be in the same enclosure left. I still have to turn the RF gain down some.
Not sure if the 8 watts is enough without some way to dissapate heat. I think I will keep it but just use some 5 watt resistors that I have in series/parallel to get the rating up a little more.
Probably would be more practical to buy a 50 or 100 watt load to use on the second radio but this was cheap to try. If anybody else tries it let me know. |
bruce
| | Posted on Wednesday, December 18, 2002 - 4:40 am: |
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QUESTION ......
Where they SHELDED from each other??? |
Jp1116
| | Posted on Wednesday, December 18, 2002 - 12:46 pm: |
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I had a piece of aluminum in between the SO-239's and soldered the resistors straight from the input to the chassis. This is an aluminum case if that makes a difference. I even unhooked the second radio from the box and moved it into another room with no wire or antenna attached and I still got a good signal. I do as someone mentioned above and just transmit into a dummyload and listen on another radio. I just got curious when it was mentioned and remembered the drawing and thought I would give this a try and see what happened, I needed a small dummy load with a pickup for a counter anyway. |
