Saturday, March 10, 2012

Cheap and Easy To Build Antennas Presentation

March 2012 ARTS Ham Club Meeting Presentation - Cheap And Easy To Build Antennas

I did the program for this past months W4CN ARTS Club meeting on "Cheap and Easy to Build Antennas".   I wanted to share how fun and interesting building your own antennas can be compared to buying pre-built antennas.  Hopefully, it will encourage others to try to do some on their own.

Here is a copy of the presentation, I gave on the subject.

The importance (not so much) of SWR Sidebar

I also did a sidebar discussion about how high SWR does not necessarily mean that much loss of power radiated in your antenna, and just how important your feed line is when it comes to such situation

As I have learned more about designing and building my own antennas, one thing I have learned is that SWR is not the most important factor.  After all, your dummy load has perfect 1:1 SWR.

For example, most people think that at a 5:1 SWR, the antenna will always consume about 56% of any power given to it and it will always return the remaining 44% to the antenna tuner, and then  point to the 44% returned power and call it lost power.   But here is the important bit – the antenna tuner says “No problem, I will just send it back to the antenna again!”.  The antenna tuner adjusts the electrical length of the antenna and coax #2 so that the reflected energy has the exactly correct phase to be re-reflected at the antenna tuner.  The first time, the tuner forwards 100 watts and gets 44 watts back from the antenna. It then forwards the 44 watts and gets back 20, it forwards 20 and gets back 9, and so on.  This goes on until finally the full 100 watts is delivered to the antenna and nothing is returned.  The real losses in all these power reflections come from traversing the transmission line, back and forth, and the losses that are inherent in the transmission line.  Except for the losses in the coax, 100% of the energy that leaves the transmitter will be radiated out of the antenna, no matter how high the SWR, because of the re-reflections.

I had created a spreadsheet awhile back to do some of my own calculations and play with the numbers to see what types of transmission line, brands of coax etc have on your output power... and I showed that spreadsheet during the presentation.  It accounts for the losses of the forward and reflected power through the feed line, which is the greatest source of loss due to high SWR.  It does not account for the loss that might be inside an antenna tuner, but from my research those are actually quite negligible compared to the transmission line losses. had an article last year that discusses some of these things as well, and tries to dispel some of the myths about SWR as well:

The formulas I used in creating the spreadsheet are not that complicated, but there are multiple steps involved in calculating the actual losses, because you have to calculate the numbers for various forward, reflected, and re-reflected signals.  First I calculate the initial loss of power going up the transmission line, based on the published db losses for that type of cable.  Then, I calculate how much of that power is radiated, and how much is reflected.  Then I calculate the losses of the reflected power back down the transmission line which gives us the amount of power that arrives back at the tuner, and will be re-reflected back up the transmission line.  The losses of the re-reflected power from going back up the coax are then calculated, and we get how much of the re-reflected power arrives back at the antenna and gets added to the power from the initial radiated power.  You could run this calculation a few more times, since there are few more reflections and you could calculate all the way out until there was no more power being re-reflected, but after the first big one, your talking about some very small portions of the initial transmission power, sort of like fractions of fractions of fractions...but for the sake of doing a little simpler math, you can assume that eventually all the re-reflected power should even out to be very close to these calculations made from the first couple of power reflections that occur in the antenna system.

If you would like to run some numbers yourself, here is a copy of my spreadsheet.


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