CSL126 said:
Hi everyone, I was wondering what type of coax is best to use for 800 Mhz. I've heard people say that RG-6 is good, but i've also heard that 9913 is good, any suggestions? Also, my scanner currently sits about 30ft in the air.
The best is Hardline or Heliax - the larger diameter the better. But, most people can't afford that. 9913 is good as long as you don't bend it. If you do bend it, you can create impedance bumps. While I won't get into the details, I'll just tell you that they are VERY bad. I tried 9913 on a system about 20 years ago, and will NEVER use it again. It's just too fragile. Not that it physically breaks - just that if you kink it (or someone else does), you may as well throw it away, and it's very easy to bend it too tight - especially in a non-tower run environment. This is not the case with foam dialectric cables.
As for raising your antenna from 30 to 40 feet, it's probably not worth it unless your antenna is shielded by something that the extra 10 feet would clear. If your antenna is already in free space, you won't notice any difference in going the extra 10 feet unless your antenna is in a null for a given transmitter. Of course, it's also possible that the extra 10 feet will move it INTO a null - decreasing performance.
The info that KA3JJZ posted is good. But, keep in mind that the figures posted for the coax loss are at 100 MHz. While the relative values are still valid, for 800 MHz you will have to multiply the loss figures by 4 (four) to get the loss at 800 MHz. Why is this important? Well, the general rule of thumb is that to gain 3 dB on an antenna in free space, you have to double the height. That means that your 30 to 40 foot increase will gain you about 1 dB or so. You will liteerally never hear the difference, BUT if you are using something like RG-6 (8 dB of loss at 800 MHz per 100 feet), your additional cable loss for that additional 10 feet will be 0.8 dB, nearly as much as your 1 dB of gain, so you will have done all that work for 0.2 dB, and there is no way you will see any difference in 0.2 dB! In fact, it takes 3dB before you notice ANY difference in the signal at all, and then it will be barely noticable. 3dB is the point at which your ear can first notice that a signal is stronger. So, was this hypothetical example worth the money and time you spent? Certainly not.
So, when calculating the upgrades, be sure to take into account the cost of the materials and the gains
and losses that you will realize. As I have shown above, using RG-6 and going up 10 feet, you will have lost all you gained. However, replacing RG-58 with RG-6 would be a good move no matter what the distance. Personally, I would go with Heliax (and at 800 MHz, I do). But if you can't afford that, go with LMR-400 or better.
You didn't say how far your total run was - only that your antenna was 30 feet in the air. Keep your feedline run as short as possible. There is a reason why some people post that their OEM scanner antenna by the window works better on 800 MHz than their antenna that is 50 feet in the air - feedline losses. If they were running RG-58/U (a common scanner feedline), and had a run of 50 feet to the antenna, plus another 25 feet in the house, that is 12 dB of loss at 800 MHz. The OEM antenna on the back of their scanner may very well have a better signal than the feedline that is loosing over 93% of the signal it's receiving!
Joe M.
, does anyone know where I can buy more 10 ft sections to add to my antenna mast? We dont have to limit this post to just height and coax, if you have any other suggestions please let me know. Thanks in advance for the help.
