EZNEC People: Question

[WA4WAX]

Joe:

Make the zepp feeder out of #14 copper stranded, insulated, save for the bottom 5 or 6 feet.  That will be bare solid copper. A spacing of 4.75 inches will be fine. Bond one end to the feed point, and place the short bar 1/4 wave below the feed point (Note: High Z feedpoint > zepp inpedance, so odd integral muliple of 1/4 wave).

Let us consider the original antenna on 40 meters.  Our zepp feeder will be about 35 feet long, with short bar at 32.5 feet.  Bond on to the feeder end to side with 1) a current balun connected to 50 ohm coax, or 2) 600 ohm (nominal) OWL running to a balanced tuner.  In the first case, the 50 ohm point will not be far above the short.......maybe ao foot or two.  For the 600 ohm case, a bit higher.  You will iteratively adjust the tap point and short bar position until you get the desired impedance.  A Nano VNA, RX noise bridge, or other suitable widget will help.

With a low Z feedpoint, the zepp feeder is 1/2 wave, or an integral multiple.  Same tuning procedure.

[KC2ZFA]

I think that article doesn't mention 15 meters for two reasons:

a) at the time it came out the 15M cw allocation was kinda recent (phone followed in 1953)

b) the tuner discussed uses manufactured push-pull plate coils and swinging links which didn't exist for 30M so as to allow the tuner to match the antenna on 15M.

[ka1bwo]

Joe:

Make the zepp feeder out of #14 copper stranded, insulated, save for the bottom 5 or 6 feet.  That will be bare solid copper. A spacing of 4.75 inches will be fine. Bond one end to the feed point, and place the short bar 1/4 wave below the feed point (Note: High Z feedpoint > zepp inpedance, so odd integral muliple of 1/4 wave).

Let us consider the original antenna on 40 meters.  Our zepp feeder will be about 35 feet long, with short bar at 32.5 feet.  Bond on to the feeder end to side with 1) a current balun connected to 50 ohm coax, or 2) 600 ohm (nominal) OWL running to a balanced tuner.  In the first case, the 50 ohm point will not be far above the short.......maybe ao foot or two.  For the 600 ohm case, a bit higher.  You will iteratively adjust the tap point and short bar position until you get the desired impedance.  A Nano VNA, RX noise bridge, or other suitable widget will help.

With a low Z feedpoint, the zepp feeder is 1/2 wave, or an integral multiple.  Same tuning procedure.

Matt from your example you will have 32.5 feet of 600 ohm line connected to the feed point of the antenna. At the other end of the line there is a shorting bar, from the shorting bar there would be an addition 2.5 feet of line for a total length of 35 feet. The impedance (Z) at the end of the 35 foot line will be a short rotated 6.6 degrees for 2.5 feet of line @7.2Mhz.  Z = .178+j73.4  ohms. The 32.5 feet of 600 ohm line to the antenna feed point is irrelevant.

[WA4WAX]

Joe:

The line below the short bar is inert, as everything is reflected back from the short bar in the line above the bar.

I recommend 35 feet to allow for some "wiggle room" for the short bar.  Start at 1/4 wave, which will be about 32.5 feet on 40 meters.

You will connect the current balun terminals ABOVE the short bar.  Try 2 or 3 feet initially.  Work up or down until you get close to 50 ohms.  Move the short bar a bit to get it right on the real axis.

For OWL and a tuner, you could probably just go directly from the feed point as the impedance of this antenna is not too far from 600 ohms.

[WA4WAX]

Here is an even better approach:

On all bands except 80, we have capacitive reactance........switch to admittance, and we have inductive susceptance.

OK........terminate the zepp feeder at 35 feet with a variable capacitor.  Bond on a few feet above the cap.  Mesh the cap to get on the real axis.  Move tap point up or down until you get 50 ohms......or 200 if you use a 1:4.  My guess on 40 meters would be a cap around 150 to 200 pF.

For 20 meters, remove about 12 feet of the feeder, and put the cap at around 18 or 19 feet.  Of course, tap point and mesh setting will be different.  Yes, we are using a hybrid transmission line stub here.  :-)

Nice way if you do not have a tuna, or do not want to use one.

Do not forget that static drain!!

[ka1bwo]

Joe:

The line below the short bar is inert, as everything is reflected back from the short bar in the line above the bar.

I recommend 35 feet to allow for some "wiggle room" for the short bar.  Start at 1/4 wave, which will be about 32.5 feet on 40 meters.

You will connect the current balun terminals ABOVE the short bar.  Try 2 or 3 feet initially.  Work up or down until you get close to 50 ohms.  Move the short bar a bit to get it right on the real axis.

For OWL and a tuner, you could probably just go directly from the feed point as the impedance of this antenna is not too far from 600 ohms.

OK Matt this explains it. You end up with a shorted tuning stub across the input feed point. With some shunt capacitance  across the input feed point, will get you to 50 ohms.

[ka1bwo]

Joe:

The line below the short bar is inert, as everything is reflected back from the short bar in the line above the bar.

I recommend 35 feet to allow for some "wiggle room" for the short bar.  Start at 1/4 wave, which will be about 32.5 feet on 40 meters.

You will connect the current balun terminals ABOVE the short bar.  Try 2 or 3 feet initially.  Work up or down until you get close to 50 ohms.  Move the short bar a bit to get it right on the real axis.

For OWL and a tuner, you could probably just go directly from the feed point as the impedance of this antenna is not too far from 600 ohms.

OK Matt this explains it. You end up with a shorted tuning stub across the input feed point. With some shunt capacitance  across the input feed point, will get you to 50 ohms.

Matt, Here's is the input impedance of your matching scheme for the RCA all band antenna for 40 meters. A 610pf  shunt capacitor was required across the 50 ohm input.  Swept 7-7.3Mhz, Vswr circle is 2:1

[WA4WAX]

Some specifics:

How far away from the antenna feed point is the capacitor? Where is the balun connected on the zepp feeder?

I envision putting the cap about 1/4 wave away, and bonding on with the balun a foot or two above it.

[ka1bwo]

Some specifics:

How far away from the antenna feed point is the capacitor? Where is the balun connected on the zepp feeder?

I envision putting the cap about 1/4 wave away, and bonding on with the balun a foot or two above it.

Using 600 ohm line, the shunt capacitor and the balun is 30.5 feet from the antenna feed point. The shorting bar is 35 feet from the antenna feed point.

[WA4WAX]

Joe:

You should also be able to find a match with a short bar alone.  In other words, a short circuited stub.

Transform the antenna input impedance to admittance. Rotate about a quarter wave, and you will be in capacitive susceptance territory.  Cancel with a small shorted stub in shunt.  Of course, you will want to cancel on the unit conductance circle. 

In practice, we have the 35 foot zepp feeder. We iteratively adjust tap point and short bar for match.

The cap is versatile if variable.  You can move a virtual short up and down the line simply by adjusting the cap.

Capacitively terminated, a hybrid stub may be modeled as the sum of the capacitive reactance and the reactance of a short circuited stub of the same length, not an open circuited stub.  Pardon this small digression.

We are getting lots of views on this thread.  I hope that means many will build this antenna.

[ka1bwo]

 

Joe:

You should also be able to find a match with a short bar alone.  In other words, a short circuited stub.

Transform the antenna input impedance to admittance. Rotate about a quarter wave, and you will be in capacitive susceptance territory.  Cancel with a small shorted stub in shunt.  Of course, you will want to cancel on the unit conductance circle. 

In practice, we have the 35 foot zepp feeder. We iteratively adjust tap point and short bar for match.

The cap is versatile if variable.  You can move a virtual short up and down the line simply by adjusting the cap.

Capacitively terminated, a hybrid stub may be modeled as the sum of the capacitive reactance and the reactance of a short circuited stub of the same length, not an open circuited stub.  Pardon this small digression.

We are getting lots of views on this thread.  I hope that means many will build this antenna.

Matt your spot on!   Using 600 ohm line, 33.42 feet of transmission line from the antenna feed point. At the end of the line a shorted 1.17 foot length of shunt 600 ohm line. The input impedance plot, across the line at 33.42 feet. Swept 7.0 - 7.3Mhz , Vswr circle 2:1, Frequency at center of the chart is 7.2 Mhz, Z= (50 +j.308) ohms.  Note: The RCA all band antenna feed point impedances was modeled from this configuration for both of Matt's matching schemes. Height above the ground for the bottom wire is 8 feet, bottom and top wire length 48 feet, each vertical wire length 16 feet.   

[WA4WAX]

Have you out one up?  If so, please give a report.  I would like to hear about night time reach on 40 meters.

Happy New Year!