Topband: Understanding or attempt to understand a flag antenna

[n4is at comcast.net]

Hi Guys

 

This concept is important and very confusing, so sorry for being a broken
record. I’ll keep it very simple to understand.

 

The Flag antenna and its variances, EWE, pennant WF  SAL  DHL and others
nice names are basic the same and like this:

 

Two wires in U shape, one upside down and one normal , with two openings.

 

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Open1    open 2

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The way to see it is like two vertical dipoles and two horizontal phasing
lines. On the opening one we terminate with a RESISTOR, the valuer of the
resistor is not important but for this exercise lets use 1000 ohms. Or two
vertical vectors and two horizontal vectors.

 

O the opening one we will connect the feed line. That’s the Flag, two
vertical in phase, Because the vertical dipole is too small it reflects the
current to the other dipole., and vice versa. The side one with the resistor
the current will be dissipated on the resistor,, but on the open 2 the
current can me connected to the feed line.

 

At this point the system could be balanced and the reactive component will
be very low if we pick for the feed line the same impedance of the load,
1000 ohms.  You can simulate that on EZNEC and you will find it very
broadband.

 

The 1000 ohms feed line is part or the receiving system and not part of the
antenna, so any loss will represent “noise” and will be add to the noise
figure of the receiver,. It you use a 9:1 transformer, the feed line could
be close to 100 ohm, It is simple to use a twisted pair close to 100 ohm
impedance , like twisted pair form a Ethernet cable will provide very low
SWR and no common mode noise, no shield is necessary.

 

The losses on the transformer and the feed like is part of the receiver
input circuit and not part of the antenna. The power noise is independent of
the valor of the resistor and the noise temperature on the vertical dipole
“open 2” is the same as any dipole. Please keep it simple and G/T of a
dipole is very well know. It is just two phased very small dipoles.

 

Based on the above is very simple to understand the EWE, just replace the
normal U by the antenna reflected by the ground (like a mirror). Cut all by
½, so the resistor will be 500 ohms and the impedance easy to match 50 ohms
using a 9:1 BALUN

 

Also the K9AY is easy to understand, just think the ground as a 1000 ohms
transmission line, and you can move the resistor close to the BALUN, like
the EWE.

 

As any antenna, the thermal noise is the limit of the amplifier to produce
any gain, if you want to work over 300 DXCC ( confirmed ) on 160m, make the
flag 24”x 12” and use it, spend time on that chair , tunning the radio and
enjoying 160, it is not easy  and that’s why we love it.

 

73

JC

N4IS

 

 

 

 

 

 

 

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