Modifying an antenna

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FireDawgEMT22

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Ok, I started a discussion in another thread earlier, and it changed over to more of an antenna feed line issues questions, so I decided this is the proper forum to bring more light on it with.

I have a PSR 600 monitoring VHF, UHF, and 800Mhz. The antenna I have is a RS 20-176 about 35-45feet up. I HAD 25 feet of RG58 running to a splitter and then from the splitter 60 ft of RG6 to the back of my scanner. I had the splitter in to attach my minitor V charging base that I added an antenna to, but I think I am just going to make a simple Dipole antenna for that.

So I removed the splitter and the RG58 and just have the 60ft of RG6. I can pick up the VHF signals MUCH better, even had to turn on the attenuator to get the signals clearly. But I am still having problems picking up the 700MHz trunked system that is broadcast from a single sight 33 air miles flat terrain away. I actually cannot get the signal to even break the squelch on tune mode.

I modified the antenna by adding two 3.5" rods into the antenna 90 degrees out from the UHF elements and it still did not help. Is there another modification I can make to this antenna or some other change to help with this reception.

I have heard alot about the Antennacraft Scantenna ST2. I have heard they are junk and stay away from them, and heard they are the best thing since sliced bread for receiving. If I am doing NO transmitting, would getting this antenna be the miracle I have heard it is?

Thanks in advance for any tips and advice you guys can give me.
 

mmckenna

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Adding dedicated radials for 700MHz was a good idea, but it sounds like they are horizontal?
If so, that would put your 700MHz radials horizontally polarized while the transmitting antenna is vertical.
The Radio Shack antenna you mentioned is VHF and UHF, so it likely won't be a great performer on 700MHz without the modification. Adding the shorter radiating elements is good, but if they are cross polarized, it won't help enough to make a difference.

As for changing coaxial cable, 60 feet of RG-6 will show 4.8dB of loss at 700MHz. LMR-400 will show about 2dB of loss at the same length and frequency. The higher grade coaxial cable will give you almost an additional 3 dB of signal to work with. All other things being equal, this would be equivalent to doubling the strength of your received signal.

That, on it's own, may help. What you could do before spending money on the new cable would be to try adjusting your antenna a bit and try to get the 700MHz elements you added in a better position. You could also try fabricating a simple 1/4 wave ground plane antenna cut for 700MHz and see if that would help.

Likely you will need a combination of a better cable as well as an antenna capable of performing better on 700MHz to get what you want. Even with the 3dB improvement in your cable and an antenna improvement, you still may not see enough signal to really do much good. If you are really hung up on getting this 700MHz system, you may need to switch to a dedicated 700MHz directional antenna.

Someone may suggest an amplifier, and that may help, but unless there is sufficient signal there to amplify, it may just raise your noise floor.
 
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