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Painted Predator Antenna
- Thread starter tvaverde
- Start date
Picky point: that gray-colored cable isn't RG-8. RG-8 has exactly the same outside diameter (0.405 inch) as RG-213. What you appear to have is RG-8X. It's smaller in diameter as you know by now, and smaller diameter cable generally equates to greater loss figures per unit length.
This is correct.
RG-8x is thinner than regular spec RG-8, and is intended to be flexible and easier to feed through small places without the need of using the very lossy RG-58 coax. It is a compromise design and cannot be compared directly to RG-8 (or RG-213 for that matter) as an equivalent. The additional losses present is why it appears to have a better SWR than your RG-213.
The DB
It is more than just the shielding, the material used between the center conductor (the dielectric), length, as well as other factors also affect this. But when all the parts come together the lower loss in the feed line will show a higher SWR, and its readings are closer to where the antennas SWR actually is as well. The exception is if there is an issue with that coax itself, (cold solder joint, kink, ect.) which is entirely possible.
Think of it this way, your radio transmits a signal, which then goes into the forward side of your SWR meter at full strength. Then as the signal travels down the feed line towards the antenna the feed line losses will attenuate the signal (or make the signal weaker). How much depends on the loss characteristics of the feed line and how long it is. When it gets to the antenna that does not have a perfect 1:1 SWR match, a portion of the signal is reflected, and travels back up the feed line. As it travels back up the feed line it is attenuated again just as before. This signal is the reflect reading on your SWR meter.
The SWR reading, being a ratio of forward power to reflected power, is affected by this loss. As the loss only affects the reflect signal, it will have a lower reading, while the forward reading will be unaffected. You can get around this by putting the SWR meter at the antenna feed point as I mentioned above.
All that being said, either of those coax lines that you used should have an insignificant amount of loss at the length that you are using. I can do some calculations and get some percentages for you if you wish.
The DB
I am going to hook up the swr meter at the antenna and see if there is any difference, I am also going to re check all my solder joints. I think I am going to cut my coax down to exactly the length I need instead of using the whole 18'. I think I can get away with as little as 12'. And I will check my SWR's between every step I make. And I will post my results.
Thanks Tony
Thanks Tony
Good deal. The correct length of coax to use is the shortest length to get from the radio/amplifier to the antenna.
18 feet of coax is advertised by some people (and some manufacturers as well actually) as some sort of magical length that you need to use. It is not. There are times when a half wavelength (about 18 feet long, go figure) is useful, but if you are only using an SWR meter for testing it will not be of benefit to you. Also, when cutting coax to that half wavelength you have to take into account the velocity factor of the coax, which that false 18 foot rule does not consider. Ironically, at 12 feet long, your coax will likely be closer to the electrical half wavelength that people refer to when they say to use 18 feet of coax...
The DB
My Sirio 145 magmount that is paired up with my Sirio Perforner PL5000 antenna has 13 feet of RG58U coax. I also emailed Sirio to ask why they selected such a length. Waiting for a reply.
Yes I spoke to Kale, and it was he who suggested to use RG8 over RG213. He also said not to worry about using 18'. He said to use just what I needed to get from point A to point B
I think Kale's advice is probably your best bet. The only reason to use specific lengths of feed line is because it 'aids' in tuning whatever is on the end of it. That usually means that whatever is on the end of that feed line needs a specific addition to it's self that it can't furnish for it's self so can't normally be "tuned" as it should be. While 18 feet of any kind of coax isn't going to magically 'cure' some problem, there's really no harm in using that length either. If that 18 feet doesn't result in a huge wad of coax somewhere, I'd leave it alone. The only thing to keep in mind is that a little 'extra' makes for easier handling, you got some slack to play around with. That stuff doesn't stretch very well, you know?
- 'Doc
- 'Doc
Still at it . Went as far as removing the paint from the antenna ( nice and shine silver). Tested the swr at the antenna. Still no luck. Best swr is 1.8-1.8 1/2.
Made a brand new bottom shaft out of 3/4 copper tubing. ( threads at coil are 1/2x20). I going to play with ground straps on the body and frame of my truck. Rite now I have cab, frame and bed all grounded together on the same corner between the left side of the bed and cab. I ran a shorter length of coax along the left side of the truck. Still no luck.
17". 22". 27".
Made a brand new bottom shaft out of 3/4 copper tubing. ( threads at coil are 1/2x20). I going to play with ground straps on the body and frame of my truck. Rite now I have cab, frame and bed all grounded together on the same corner between the left side of the bed and cab. I ran a shorter length of coax along the left side of the truck. Still no luck.
17". 22". 27".
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