ATAS-120 as a base antenna

[rz6yf]

Today, I have installed ATAS-120a outside as a main HF antenna for FT-857D.
When done with tuning, 857D says "HSWR"(high SWR). Checked on 7&14mHz.
Same(HSWR) result. Checked from outside while tuning, antenna size changes. What's wrong? Installed as "it is" supplied, I've got a steel pole with satellite tv dish and ATAS on top of the pipe. Antenna isn't grounded. May this cause the problem ?

 

[gary in ohio]

Today, I have installed ATAS-120a outside as a main HF antenna for FT-857D.
When done with tuning, 857D says "HSWR"(high SWR). Checked on 7&14mHz.
Same(HSWR) result. Checked from outside while tuning, antenna size changes. What's wrong? Installed as "it is" supplied, I've got a steel pole with satellite tv dish and ATAS on top of the pipe. Antenna isn't grounded. May this cause the problem ?

You forgot to install the other 1/2 of the antenna!!!! First, the AT-120 is a heavy compromise mobile antenna. Its gives you a good SWR and some of your signal gets out but its not a great antenna mobile or otherwise. Second, When mounted on a vehicle you provide the antenna with a large groundplane, the car body. In your portable config you dont have that. Think of a dipole, it has 2 1/2's, one side to ground one to feed. What you need to do is put some radials under the antenna. a couple per band if possible. As for grounding you should put a seperate ground on the antenna as well.

 

[rz6yf]

You forgot to install the other 1/2 of the antenna!!!! First, the AT-120 is a heavy compromise mobile antenna. Its gives you a good SWR and some of your signal gets out but its not a great antenna mobile or otherwise. Second, When mounted on a vehicle you provide the antenna with a large groundplane, the car body. In your portable config you dont have that. Think of a dipole, it has 2 1/2's, one side to ground one to feed. What you need to do is put some radials under the antenna. a couple per band if possible. As for grounding you should put a seperate ground on the antenna as well.

Thank you for replay!
So,what exactly should I do (technicaly) to get the missing 1/2 part of antenna? I saw somewhere, a guy installed same antenna on the hotel's balcony being on business trip....

 

[gary in ohio]

Thank you for replay!
So,what exactly should I do (technicaly) to get the missing 1/2 part of antenna? I saw somewhere, a guy installed same antenna on the hotel's balcony being on business trip....

You need to put a car under it or at least emulate one. The hotel balcony would have been most likley a metal rail 10-20ft long. that would have emulated the car. You need something similar. Connect 1 or more wires to the frame of the antenna and run them horizontal. You may find more wires and wires 1/2wave long on the freq you want to use works best, but anywire 20ft or so long is a good start.

 

[w9awx]

Do you have the base mounting kit for the antenna? The antenna needs a good ground plane to work off of a vehicle. The base kit gives you the ground plane you need to have it work off the vehicle. I am looking at the same thing for my truck and home too and have been told many times that this radio needs a very good ground plane. Guess you discovered that the hard way. But, hey. We all learn from everyone else experience.

73
Gregg

 

[rz6yf]

ATAS-120: Any formula available?

You need to put a car under it or at least emulate one. The hotel balcony would have been most likley a metal rail 10-20ft long. that would have emulated the car. You need something similar. Connect 1 or more wires to the frame of the antenna and run them horizontal. You may find more wires and wires 1/2wave long on the freq you want to use works best, but anywire 20ft or so long is a good start.

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I tried what you said above. Here's some interesting results I've got:
Let's switch to metric system for easy understanding. OK, I want to tune atas -120 on 40m band. 1/2 wave is 20 meters of wire, 1/4 wave is 10 meters. I decided to start with 20ft(6 meters), just like you said.
Antenna tuned on 11,12,13,14 Mhz with this one. Wire 6 meters long is almost 1/4 of wave. So, I figured, if I want to tune antenna to lower frequency, I will need to increase the lenght of the wire.So, increasing the lenght to 12 meters antenna tuned at 19-21 mHz. Why is that? Is there any formula exist to calculate how long a ground plane(?) suppose to be? And, how many ground planes I suppose to have? One for each band or one universal? Thank you, guys!

 

[gary in ohio]

Your adding radials more than adding a ground. Your getting the lengths correct, but you need 2-4 radials per band. laid out under the antenna.