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.

Eham.net 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: http://www.eham.net/articles/26720

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.

440 MHZ(70cm) Preamp

I completed building my 440mhz preamp.  The main reason I built this was to use for AMSAT reception of the amateur radio satellite downlink signals.

The hardest part with working the satellites is acquiring of the downlink signal from the satellite.  That is much harder than the satellite hearing you.

Using a small pre-amp can make a huge difference in being able to establish communications on the satellites.

I chose to build one using the cheap and easy to build kit available from Ramsey Electronics.  They have cheap pre-amps for 2m, 220mhz, and 440mhz.  Each one is only $14.95.  They use a  2SC2498
transistor and provide high gain and very low noise.  They have a tuned filter and provide amplification over about 24mhz of bandwidth.

They are popular with the people on the AMSAT mailing list, where they are said to compare favoribly to commercial pre-amps 10-20x the cost.  Measurements reported on the mailing list indicate performance of 20db of signal gain, with less than 1db of noise gain.

A pretty cheap kit for the usefulness it provides. Here is the page on their website for more info.

I chose an old tin box I had to house the finished kit.  I save all the tin boxes I can find such as Altoids, and any that come with gifts or whatever.  This particular box was from a cartridge that my kids got for the NintendoDS.  Its roughly twice the size of an Altoids tin.  The size was really needed to house the batteries.  In order to keep things simple I used a pair of 9V batteries to power the device.  They are wired in series and provide about 18V.  Even though the specs say to use 15V, it seems to work just  fine with the 16-18V the two 9v batteries in series provide.  The twin batteries should provide a long battery life between changes as well.

The first thing I did was to start to mount the external connectors.  This consisted of a pair of BNC Jacks, and a SPST switch which will be used for the power ON/OFF.

I have become a fan of using various metal tins for my kits and projects.  I have discovered one invaluable tool to use for the metal work is a metal hole punch.  I bought a cheap one from Amazon which comes with dies for making various size holes.  It works quite well, and makes very neat holes in the tins.  I definitely recommend it if your going to be using Altoids tins and similar boxes for any kit building.

Once the external connections were made, I mounted a couple of 9V battery clips inside the box using a bit of JBWeld.

Then I mounted the finished board, and made all the internal wiring connections.  The kit board looks small in this box big enough for the two 9V batteries.

Added a some labels from the ole label maker on the cover, and its complete and ready to go.  Doing some testing with the amp, its sounds quite good.

I plan on using this with my ARROW Antenna, and maybe velcro mounting somehow to the tripod or something similar.  I am also considering adding a LED to the circuit to show the power is on, in order to help visually provide a assistance to try and keep from forgetting to turn it off.

UPDATE:

It is also important to note that since this has no TX/RX switching built in this is for RX use only.  I built this primarily to use for working on amateur satellites.  In order to use the AMSAT's you transmit on 2m and recieve on 70cm.   The Arrow Antenna is actually two separate (2m/70cm) yagi antennas on a single mast, and each antenna has its own feed line connection.

Using a Comet CF-416 diplexer, I connect those antennas to the same radio, or sometimes I actually use two radios and have one for RX and one for TX.. this lets you hear yourself echo on the satellite repeater.  

Here is how it would look in a block diagram.  Even though the arrow is a single antenna mast, I show it broken out into two separate antennas in this drawing.

Just for reference here are pics of the Arrow Antenna and the Comet CF-416.

The OTHER option for working the satellites is to scrap the diplexer and just use 2 HT's or an HT and a scanner.  One HT will be connected to the 2m feed line and used for TX, and the other will be connected to the 70cm feed line and used for RX.  The advantage here is you can monitor your signal and listen to yourself bounce off the satellite.

This is what that setup would look like:

ARRISAT-1 Heard

Was able to hear ARRISAT-1 this evening with my 2m ladder line J-pole vertical up about 50ft in a tree.  You can hear the audio fairly well, except for the static crashes and noise in and out.

I will have to pull the ARROW antenna out and try to get a SSTV image soon.   You can hear some of the SSTV transmissions in between the voice segments.

Here is the audio recording I made of that pass.