You are actually feeding a collinear antenna with two 3/4 wave legs. Thus it is being fed at a low impedance point, just like a normal dipole. Adding another 1/4 wave element like the stub means that you will then be feeding two full wave legs and that will mean you are then feeding at a high impedance point.
I proposed adding a matching stub because the simulation results of the horseshoe show a very high SWR of 3.5 (z = 178- j4.8)
I dont know why, but the simulation results defy your logic.
The best way to find out is to make the antenna, first without stub, then if results are not satisfactory, add a stub and see if it improves the performance.
The unknown factor is the bending of the elements, but the electrical reality of current/voltage points is beyond doubt. Hopefully the attached impedance diagram makes it clear. The 3/4 vertical antenna (more commonly a physical 5/8 with matching section) is one of the oldest VHF antennas around.
Not a dig, but it wouldnāt be the first time that the modelling has thrown up odd results.
It is not only element length which determines the impedance. Their configuration and bends also drastically affect impedance.
Below is an example of two 1/4 elements configured in 3 different ways, resulting in drastically different impedances.
CLICK ON IMAGE TO SEE LARGER SIZE
I fail to see the relevance of your examples to the antenna discussed. Never mind, carry on modelling 
I dont depend totally on modelling, it is at the best a guide. The things to be trusted is actual measurements by a VNA, and field test of a prototype to verify performance
There is no relevance between my examples and the antenna discussed. The examples are about your statement that impedance dependent only on element length, irrespective of configuration.
1 Like
The stub in your drawing looks like it is shorted near the feed point, is that intended?
It would be good if you could construct these antennae and test them on your VNA and field test them to verify their performance.
As i understand 450 ohm ladder line is designed as transmission line to transfer power from one point to another with minimum loss. By design there should be be no radiation.
The half wave folded dipole should still be a half wavelength from tip to tip and not a half wave dipole folded over and joined together. The characteristic impedance depends on such properties as element thickness and spacing and 300 ohm is a special case.
S.
Transmitting insects? 
Yeah, those British/Aussies and their weird language 
Antennae vs. antennas
In the U.S. and Canada, the plural of the noun antenna is antennae when the word denotes the flexible sensory appendages on insects and other animals. But when the word refers to a metallic apparatus for sending or receiving electromagnetic signals, American and Canadian writers usually use antennas . British writers tend to use antennae for both purposes. Australian and New Zealand writers are split on the matter, using both plurals for the metallic devices.
Yes, the short-circuit at the end of stub is intentional.
Impedance matching can be done by either a āshorted quarter-wave stubā or by an āopen-ended quarter-wave stubā.
Iāve seen the word aerial used too.
Should work like a champ.
You might have to move the feedpoint up or down a bit to yield get optimum results.
(similar to the way a J-Pole is tuned)
1 Like
.
I have already shown my intention to make and test it next weekend in my earlier post
.
.
Yes, a transmission line has nearly zero radiation, but the subject of discussion here is NOT radiation efficiency, it is āimpedance presented by a pair of elementsā, and for that purpose a ladder-line is also āa pair of elementsā and serves as good example of impedance depending on configuration. Even changing spacing between two elements changes impedance:
Quote from Wikipedia Twin-lead
" Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin-lead, was once widely used to connect television sets to their receiving antennas. The 300 ohm twin-lead for television installations has been largely replaced with 75 ohm coaxial cable feedlines. Twin-lead is also used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals.
The characteristic impedance of twin-lead is a function of the wire diameter and its spacing; in 300 ohm twin-lead, the most common type, the wire is usually 20 or 22 gauge, about 7.5 mm (0.30 inches) apart. This is well matched with the natural impedance of a folded dipole antenna, which is normally around 275 ohms."
The 300 ohm Twin-lead used earlier with TV
.
The 450 ohm Ladder-line
.
The 600 ohm Open-wire Line
Was just curious because you didnāt note the length beyond the feed point and i imagine it is critical
Yes, it is critical and to be initially determined by simulation (an aproximation), then finally by trial-and-error by adjusting the tap point up and down.
I have used this stub with 1090 Mhz Franklin antennas, and found that the optimal position of cable tap is about 15 to 20 mm from the shorted end. This distance may be somewhat (but not much) different for the Horse-shoe antenna.
True.
I have used the impedance matching stub in the J-Pole and in the Franklin I have made some years ago. I found the aproximate optimum point by simulation, then accurately by trial-and-error by moving the cable tap up and down. For my build of both the J-Pole and Franklin, the optimum point was between 15mm and 25mm from shorted end of stub.
From Archieves - My Old Antennas With Impedance Matching Stub
1 of 3 - J-Pole
.
2 of 3 - Super J-Pole
.
3 of 3 - Franklin
Just thought Iād provide an update. Ductenna has been running for almost 3 weeks pointed mostly southward out a second floor window. Gain remains set at -10 to maximize plane counts over nearby messages. My rank has improved over the spider from mid-600s to 498 today, so itās clearly an improvement (or everyone above me is degrading due to the super secret bug that I cast out to all FA receivers 
). I never added a second duct section (mostly because I forgot about it by the time I had to run up to Home Depot), so I know my SWR could improve but Iām also liking how omnidirectional it is at the moment.
Hereās what it looks like now with signal strength coloring:
1 Like
@GDoggRaspberry
Glad to know ductenna is working great for you.
@xlr99
In fact aerial is the norm in UK and most commonwealth countries. I rarely see a Britisher write antenna.
1 Like