On Sun,4/3/2016 8:33 AM, Don Butler wrote:
Barry . I just tested my 20 meter and 17 meter whips by fully extending them and connecting them directly to a Rig Expert AA-230 Pro outside in the open yard ... With NO Counterpoise wire ... this is the first time I've tried this ..
Good that you've never done that -- as W0MBT noted, omitting the counterpoise leaves out half of the antenna! The counterpoise provides a RETURN for antenna current. Without that return, the chassis of whatever box it's connected to (and whatever the chassis is connected to) serves as the return.
Let's get something straight. Current flows in LOOPS, whether it's DC, audio, or RF. If current flows out on one wire (in this case, the whip), it MUST flow in on something else connected to the transmitter. In the case of the whip, it's whatever is connected to the chassis of the transmitter. We call that a counterpoise. An antenna creates an electromagnetic field, which space couples between the two pieces of the antenna (the whip and the counterpoise), so around the antenna space is part of that loop.
Without that counterpoise, the antenna doesn't work as well. First, it is less efficient, because the return path for the feed current and the field is poor. Second, because it is part of the antenna, a missing or shortened counterpoise causes the antenna to resonate at a higher frequency.
Radials are a more ideal form of counterpoise -- they SHIELD the antenna from the lossy earth (which burns TX power before it can be radiated); instead, return current flows in low resistance radials, making the antenna more efficient (and our signal louder at the other end).
On Sun,4/3/2016 9:17 AM, Wayne Burdick wrote:
This probably goes without saying, but... always use a counterpoise (at least one wire) if you're actually trying to make QSOs. Without one, the whip not be resonant on its target band, and you'll lose 15-20 dB of signal at the receiving station.
Exactly.
Even with the counterpoise, I always use the internal ATU in the KX3 to improve the SWR.
Poor choice of words. An antenna tuner does NOT "improve the SWR" -- SWR is a property of a TRANSMISSION LINE, and it is determined ENTIRELY by the match between the antenna and the transmission line. Rather, an antenna tuner transforms the Z of the antenna to the load impedance that the output stage of the rig wants to see as a load, in most cases, 50 Ohms resistive. This does two things -- it prevents protection circuitry from "folding back" the output stage (reducing drive to the output stage, thus reducing its power) to protect it, and it reduces distortion in the output stage.
For listening only, I'll sometimes use the whip without a counterpoise wire. I still use the ATU in the KX3 in this case (you can't hurt it, since this is done at 2 W). This resonates the whip for RX purposes.
In this context, the counterpoise (and the tuner) do two things -- they make the antenna more efficient (by providing a better return for current), AND increase the current at the input of the RX. With TX, we care about efficiency of the antenna (how much RF field does it create) because it makes our signal stronger. With RX, we don't care much, or don't care at all, because we have more than enough sensitivity in the RX to hear weaker signals. SO -- a counterpoise is far less important for listening.
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