VK2RK
Active member
The above was taken from the Codan Brochure
In my quest to find an antenna that provided the best advantages over disadvantages took me down several rabbit holes with mixed results.
I ran for some time a sort of fan dipoles on 80, 40 and on 30, the lot gave me a mix to tune with the aid of a tuner from 3.5 to 50 MHz
however the efficiency was not the best when operating on the harmonic frequencies but did provide some good contacts.
I did try an off centre fed dipole, the near objects to the antenna made the tuning impossible, this was an interesting tour of the rabbit warren as it provided several empirical facts, one do not use a metal centre pole if intending to install in an Inverted V configuration, expect high common mode currents as the matching will not be equal along the operational band and bandwidth, this antenna gives the impression of been a good solution as far as working all bands however the near object and high common mode can be a challenge to mitigate as the feed point impedance swings widely making the BALUN design very critical as the choice of ferrite properties.
In conclusion the humble centre fed dipole gave the best trouble free performance.
For sometime a friend kept pushing me to look at a travelling wave antennae, information on these antennae's are hard to find more so for particular designs, so to try and understand on a theoretical point of view is a challenge. However this kind of antenna system has existed back in the early days of radio, H.H Beverage in 1921 came up with his system of travelling wave antenna, then we have the Rhombic that is also of the same family, there are many variances also now used in the higher frequencies encompassing microwaves.
One fact emerged from my search is that all commercial manufacturers use travelling wave antenna systems, the reason for this is the flat frequency response making antenna tuning unnecessary.
Been a sceptic and a believer for a long time that high Q systems with the resulting narrower bandwidth provided the best signal to noise ratio, providing some antenna gain, but I ignored problems that to extend the bandwidth usually a tuner is required, (OLD Pi Matched this is no problem) One fact often ignored (by me as well) is that the moment that one shifts away from the centre frequency the system is no longer resistive becoming reactive, the further from this point and the feed point impedance changes to a point that upsets things like the BALUN along with the feed line etc, this then in turn creates other problems usually high common mode currents, the return loss is mitigated by the tuner but the common mode is not, so the further one goes away from the centre frequency the worst things get, having said this I have had good success with my resonant dipoles once I fitted many ferrites to mitigate common mode at the feed point.
Travelling Wave advantages
This kind of antenna system is not resonant to any of the frequency range it's designed to operate within.
Has a flat VSWR,
No Common MODE currents
Can be fed with 450 Ohms open feeder (Ladder Line)
Very wide bandwidth depending on design 3MHz to 30MHz (VSWR < than 1:1.5)
Operates in the H field theoretically should pick up less man made noise (Jury is out on this one, my location is low noise so can't say)
Size of antenna is smaller that a 40 Meter dipole
Can be installed Horizontal or inverted V
Inverted V gives the best omnidirectional properties.
First Impression
The angle of propagation seems to be a bit higher at the same height of a resonant dipole , but this is an early day impression, according to the good oil from people like CODAN that say the height above the ground is not critical, but they are not looking at the distances we do thus can make this kind of claim.
Overall my impression is a positive one, I managed an overseas contact on 10 and 40 meters under bad conditions after the solar flare
Comparing my MOXON on 20 Meters with this antenna showed the gain of the MOXON to be approx. 11 dB gain, this is reasonable that shows the gain of the travelling wave is equal to a resonant dipole. Note I did this test under the bad RF conditions due to the solar storm.
Conclusion
If you are after a wide band antenna why would you consider a G5RV or an OCFD antenna, putting up with all the issues created.
The travelling wave antenna allows you very wide frequency range with no need to have a tuner.
What else can I say, I AM SOLD.
These can be home built using common available materials or can be purchased ready to go out of China
Only issue with the Chinese units that rate the antenna at 200 Watts is the supplied 9:1 BALUN that in my opinion will not stand a continues 200 W signal as the ferrite used is on the small size likely to heat up.
I built my own 9:1 BALUN that is good for 3 KW and that is what I am using with the 100 Watt terminating resistor, so far with 400 W on SSB mode the resistor is taking the load. I expect it to fail any time soon thus have on order two 200 Watt resistors that will be installed in parallel making the antenna good for well over a KW
Attached file is a Power Point presentation on the theory of Travelling Wave
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