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HF9V Tuning - 50-ohm or 75-ohm with RigExpert AA-54?


Nolan - KI5IO
 

I'm planning on doing initial tuning of my new HF9V outside at the coax feedline connection from the 75-ohm matching coax.

I'll be using my RigExpert AA-54 that has the option to switch between 50-ohm and 75-ohm for testing.

I'm assuming (could be a not great decision) that I should select the 75-ohm option on the AA-54 seeing as how I'm looking at the antenna through the 75-ohm matching coax??

Or ... would using the 50-ohm option be the preferred choice on the analyzer?

Doing a validation of the HF9V tuning from in the shack and looking through the 50-ohm feedline I would plan on using the 50-ohm option on the AA-54.

So ... I think I'm "over-thinking" this, but wanted to toss it into the mix looking for input.

--

73  -  ki5io
Nolan Kienitz
Plano, TX


Dennis W0JX
 

The 75 ohm cable is a transformer that brings approx 100 ohms down to approx 50 so I would use the 50 ohm setting. I have an AA-55 Zoom here and it is a fantastic instrument.

73, Dennis W0JX

On Thursday, September 10, 2020, 08:53:20 PM EDT, Nolan - KI5IO <ki5io@...> wrote:


I'm planning on doing initial tuning of my new HF9V outside at the coax feedline connection from the 75-ohm matching coax.

I'll be using my RigExpert AA-54 that has the option to switch between 50-ohm and 75-ohm for testing.

I'm assuming (could be a not great decision) that I should select the 75-ohm option on the AA-54 seeing as how I'm looking at the antenna through the 75-ohm matching coax??

Or ... would using the 50-ohm option be the preferred choice on the analyzer?

Doing a validation of the HF9V tuning from in the shack and looking through the 50-ohm feedline I would plan on using the 50-ohm option on the AA-54.

So ... I think I'm "over-thinking" this, but wanted to toss it into the mix looking for input.

--

73  -  ki5io
Nolan Kienitz
Plano, TX


 

Nolan,
The input to the 75 ohm matching section is actually 50 ohms.  It is designed that way to match 50 ohm coax.  Please be aware that the feedpoint impedance for a standard resonant 1/4 wave vertical is actually around 34 ohms.  That matches to 50 ohm coax with a minimum (but not 1:1) SWR.  The matching section is only for the 20 meter band where the antenna is actually 3/8 wave long and that feedpoint impedance (at the base) is actually about 100 ohms.  However, your SWR may appear very low if your radiation resistance is high.  If you have just a few radials, it is possible to have several ohms of resistance in series with the antenna.  A poor radial system may present 40-45 ohms at the input to the transmission line.  With all of the additional resistance in the circuit, the Q of the antenna goes down and the bandwidth gets wider.  A very good radial system will give you 60-80 kHz bandwidth on 80 meters and only about 300 kHz on 40 meters.  The bandwidth on 20 meters is very broad (beyond the band limits actually) because the feedpoint impedance is so high.  The antenna works across all of the bandwidth on the other bands except 6 meters if you have that on your antenna.
--
Al
WB9UVJ


Al AB2ZY
 

On Thu, Sep 10, 2020 at 06:46 PM, Al WB9UVJ wrote:
However, your SWR may appear very low if your radiation resistance is high.
Not quite sure what you are trying to say here. Radiation resistance is a good thing, the higher the better. Its effect on SWR is pretty meaningless, the important thing is efficiency. A simplified way to think of a vertical (any, actually) antenna system is a lumped impedance consisting of the sum of radiation resistance, component resistance, and ground loss. Power is proportionally divided among them, so the higher radiation resistance is relative to the loses, the more power you actually transmit,

Al
AB2ZY


 

Al,
Any resistance that does not radiate is bad.  In this case, 10-20 ohms of resistance in the radial field makes the SWR look better because the load appears to be closer to 52 ohms then the pure radiator impedance of 34+ ohms.  The loss is real resistance and hence it eats transmitter power that won't be radiated.  And yes it affects efficiency as well.  As I said, the added resistance also affects the antenna Q and not in a good way.  As the resistance goes up, the Q goes down and the bandwidth increases.  But it is all a lie.  If the antenna at resonance produces a 34 +j0 ohms and the radials produce 20 ohms, then almost half of the transmit power is going to heat up the wire.  If the antenna were to have a higher radiation resistance, like our antenna at 20M (~100 ohms) then the 20 ohms of radial resistance consumes less of the transmitted power. 
--
Al
WB9UVJ