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Comet DS-150S Not Doing Well

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drayd48

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#1
I have recently bought a Comet DS-150S from The Antenna Farm in hopes that I would improve my scanner reception on P25 800. I am trying to pick up a 150 watt system from about 13 miles away and pick it up OK on my scanner with a RS 800 antenna on it, but I get nothing on it with the discone. I tried replacing the cable on it and it did just a little bit better but it is still really bad. It picks up GREAT on VHF and the other very close P25 system really close to me but I can pick up the other one. Is there anything I can do to improve my reception? Thanks
 

drayd48

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#3
I am running 75 Feet of 5GR8X I think. I replaced the cable that came with it after it was bad and it improved a little bit but now very well on 800. Works great on VHF but now 800. Can you suggest a good antenna for picking up 800 9600 signals from far away? Thanks
 
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#4
Discones are pretty simple antennas, so I'd recommend doing some more troubleshooting before replacing it. Since you haven't completely ruled out other issues, replacing the antenna might not fix the issue.

So, from what i gather above, you get some reception of the system with a RS800. That should mean there is something useable there. Ideally, an outdoor antenna should work better.

There are cases on simulcast systems where too much reception can work against you. If you could tell us exactly what system you are listening to, that would help.

Not sure what "5GR8X" is, I think you might be referring to RG-8X.
RG-8x isn't going to work well at 800MHz. At those frequencies, and at 75 feet long, you are losing almost 90% of your received signal just in cable losses. Considering that discone antennas exhibit zero gain, that means what little signal you are picking up, only 10% is making it to the radio.

10% isn't a lot to work with. It isn't surprising that the RS-800 is working better.

Since coax cable losses decrease as frequency drops, that would explain why the antenna appears to work better on VHF, much more signal is making it through the coaxial cable to your radio. Using one of the many on line coaxial cable calculators, at 154MHz, almost half the signal is making it to your radio.

So, since the antenna probably isn't the issue, replacing it would only work if you had a big yagi directional antenna that picked up the signal so strongly that it was enough to get a usable signal to the radio.

That would work, but you'd sacrifice everything else in the process. A directional antenna like a Yagi is good only on a narrow slice of the spectrum and really only in one direction. Probably not a good solution for a scanner listener.

What you need to do is replace the coax cable with something shorter and/or of higher quality.

Reducing the coax cable length would help, if that is an option. If it isn't, you'll need to replace the coaxial cable.

For 800MHz, you really need to be using much better grade cable.
Minimum I'd recommend for a 75 foot long run would be Times Microwave LMR-400. That'll have less than 3dB loss at 75 feet and 854MHz.
It's a bit stiff, so it'll be harder to run. It won't bend as sharp as the smaller cable. It's also going to be more costly.

Depending on what your budget is, you might even want to go to a higher grade cable:
Times Microwave LMR-600 would be a better choice. It'll have 1.8dB of loss at 854MHz/75 feet. That would mean 65% of the signal would make it to your radio.
Drawback of LMR-600 is it's much stiffer than LMR-400, needs a big bend radius and is going to cost more.

A step even farther up would be LDF4-50A Heliax cable. Expensive, unwieldy, but 1.5dB of loss will mean 70% of the signal will make it to the radio. Overkill, yes, but options are nice to have.

Another option to consider would be to leave your discone as is with the existing coax and use it for VHF use.
Install a new dedicated 800MHz vertical antenna with a few dB of gain and feed it with one of the cables listed above. That will really improve reception, but will require switching antennas depending on what you want to listen to, or using a diplexer.

I think your first step would be to replace the coaxial cable. Use the LMR-400 at minimum, better if you can afford it. Use that with your discone and you'll see an improvement.
Get the cable with the connectors already installed. Unless you've done it before, have the right tools, it'll save you a lot of headaches.
Get the antenna end with the right connector to match your antenna. Don't use adapters. Make sure you waterproof the connection at the antenna properly.
At the radio end, don't connect these heavier coax cables directly to the radio. These cables are stiff enough to cause damage.
Instead, terminate the cable with a female type "N" connector. These are good, low loss connectors.
Use a short jumper with a male type "N" connector on one end and a connector to match the radio on the other. Use something like RG-58 or similar. For a short run, there won't be enough loss to worry about. The flexible jumper will prevent a lot of strain on the radio.
 
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#5
+1 On upgrading your coax as was suggested above. Also, if you have a handheld scanner, you could take it with you to the roof and connect it to your antenna with a shorter length of coax. Just make sure you aren't too close to the antenna that your blocking reception. If you signal at 800mhz improves, that's a good indication that you're experiencing loss through your cable.
 

ko6jw_2

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#6
If you can receive with the RS antenna, then the discone should work just fine. The problem is in the feed line or the connectors. If you're not experienced in attaching connectors, I suggest getting a commercial feed line using LMR400 or RG213 etc. with the connectors already attached. The Diamond DJ-130NJ is a better discone for uhf since it uses N connectors. The Comet has UHF connectors which, despite the name, are not good for UHF. I have done side by side tests and get much lower SWR at UHF frequencies using N connectors. The other plus is that they are easier to attach than UHF connectors once you get used to them.
 
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#7
While I certainly don't disagree with any of the above comments regarding coax type and lengths, there may be a more economical path that you could take that MAY work for you at your location. You didn't state what your budget was for this project so I figured I would provide you with the following alternative information.

First off I looked up the attenuation at 900 MHz for the RG-8X coax that you currently have (supposedly), the LMR-400 being recommended and standard RG-6 cable TV coax. Here are the attenuation numbers for 75 feet of coax:

RG-8X: 9.5 dB loss
LMR-400: 2.9 dB loss
RG-6: 4.5 dB loss

From those numbers you can see that LMR-400 is clearly the best choice of the three from a loss standpoint at 900 MHz. You can also see that RG-6 cable TV coax has less than one half the loss of your current RG-8X at 900 MHz while the LMR-400 has less than one third the loss of your current RG-8X at 900 MHz.

If you (and your budget) are looking for something more economical and easy to work with you might want to consider using RG-6 coax to replace your RG-8X. It has the advantage of being readily available and you can terminate the ends with standard F connectors (compression connectors preferred) and then just use the appropriate F adapter at each end of the coax. At the scanner end it would probably be an F-to-BNC (depending on your scanner) and at the antenna end an F-to-PL-259. Any additional loss with the use of adapters will be negligible.

As has already been suggested above, you would benefit from keeping the length of your coax to an absolute minimum to reduce loss. That advice would apply to whatever coax type you decide to use.
 

ko6jw_2

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#8
RG-6 is often mentioned in these threads as being a low loss and inexpensive option. It is 75 ohm coax and the antenna and the radio are 50 ohms. There is a loss from the impedance mismatch. I know many RR members will not agree, but it is true. Also, F connectors were originally designed for TV and FM broadcast antenna lead ins. There are not very good at UHF frequencies. The adapter needed is also an impedance bump. I expect there will be responses from those that will disagree. So be it.
 
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#9
While I'm not an advocate of using RG-6 for scanners, the performance of the cable and associated F connectors is very good through several GHz, if you really need a 75 ohm feedline. The newer breed of compression connectors and adapters like double female with the blue insulation is rated to 3GHz. I've swept many brands of compression F connectors and they measure as good or better than BNC types through 2GHz for insertion loss and return loss.
prcguy

RG-6 is often mentioned in these threads as being a low loss and inexpensive option. It is 75 ohm coax and the antenna and the radio are 50 ohms. There is a loss from the impedance mismatch. I know many RR members will not agree, but it is true. Also, F connectors were originally designed for TV and FM broadcast antenna lead ins. There are not very good at UHF frequencies. The adapter needed is also an impedance bump. I expect there will be responses from those that will disagree. So be it.
 
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