Desclaimer: Articles here are purely based my personal experiments and observations. Though i read lot of articles and books on my favorite subject of Antennas and Propagation, i am no expert in this field. If you are here looking for such accurate information, this blog may not be for you. Otherwise, hope you will enjoy reading these posts and probably try some techniques yourself!
I have been using HF multibander for many years now. My first multibander 40-10m antenna at Mumbai QTH involved simple inverted Vee with single feeder at around 20ft above the apartment terrace. It worked very well giving hours of fun on the band.
I started using tuned magnetic loop for QRP digital modes when I traveled within US. It was simply amazing experience using loops.
When I moved to Bangalore in 2011, my first comeback antenna was Hex beam. It is awesomely portable, and gave value for every bit i spent. It was my sole antenna for couple of years with some experimental wire delta loops, dipole and verticals on the side. Currently, I have Spiderbeam antenna which is another dB upgrade to my station for sure!.
All these times i was not much having thoughts around monoband antennas other than my 40mtr vertical. I had not even considered putting up any serious yagis due to apartment living. Reading a lot about other contest stations around the world, i was wondering why they normally have mono bander stacks. One obvious reason is for stacking antennas with optimal height distances and pattern uniformity. Second reason, i am sure these multi stations aim is for reducing harmonic radiation.
This second aspect, never caught my attention till now. Here is what happens to typical station transmitting with solid state amplifier. Suppose amplifier's LPF section is not working, and fundamental on 40mtr is around 400Watts, second harmonic is 40dB down and third is only 10dB down. Refer https://www.w6pql.com/a_1.5kw_lpf_for_160-6m.htm
15mtr band will see 40W signal for 400W. Without LPF between amp and multiband antenna 40 to 10mtr fan dipole for example, will transmit both 400W fundamental signal and also 40W harmonic signal will be transmitted on the air. This power is more than sufficient to cause QRM. It just sucks!.
Currently my amplifier has LPF board which suppresses the harmonics to greater than -50 dB. That is to teen milliwatt level which brings signals into safe zone. Now going back to multiband antenna again assuming that there was no harmonic suppression, with yagi of 10dB gain, 40W harmonic would sound like 400W signal simulcasting on harmonic band. With resonant monobander this problem is eliminated. In reverse direction it is true for receiving as well, resonant monobander acts like pre selector eliminating any front end related issues and IMD. Whereas, multi bander keeps gate open for all signals resonant on those bands.
This topic started to make me think some field antennas tried in the past by me such as fan verticals and fan inverted Vee, may not be right choice during the contests or in multi station activation.
A point to ponder further and probably test sometime!
Cheers
Kiran VU2XE
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