Category Archives: Radio

Helium, LoraWan, Radio

Antenna Gain vs Elevation

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Radiation pattern for Larid FG8246

A lot of people don’t realize that high gain antennas achieve their gain by narrowing the vertical beam of the antenna.  For instance the Laird FG8246 antenna is listed as having  6 dBi gain at the horizon, but it drops to no gain at +/- 20o angle off of horizontal (AOH) and drops even more beyond that angle.

So, what does that mean in real life? Well, let’s imagine that you have that antenna mounted on a 100′ tower. The 20o beam will touch the ground roughly 300′ from the base of the tower. The formula to calculate the horizontal distance from the base to the intersection is:

Height * sin( 90 ) / sin( AOH )

In the diagram above, the area below the blue triangle will be limited in connectivity. If there are radios in that area, they may be “overlooked” by a high gain antenna, especially if the antenna is mounted on a high tower on the top of a mountain. You can use the formula above to calculate the distance from the tower the antenna should be most effective.

HALF POWER BEAM WIDTH (HPBW)

This is a specification you may see on some antennas.  It is the angle where the antenna is at 1/2 of it’s maximum gain.  So for instance, a 8.15 dBi antenna should have a HPBW of 19.5o (or 9.75o AOH).  Basically, a 2.15 dBi antenna has a HPBW of 78o and for every 3 dBi of gain, the HPBW is cut in half. 

Maximum GainHPBWAOH100′ Tower500′ Tower
2.1578o39o159′795′
5.1539o19.5o300′1500′
8.1519.5o9.75o590′2950′
11.159.75o4.9o1,170′5850′

Where more is less….

Ok, now let’s look at a situation where having a higher gain antenna could SERISOUSLY hurt your coverage. Imagine a 100 story building in the middle of NYC. On top of that building they have a 11.15 dBi gain antenna on a 20′ mast. Assuming an average of 14′ per floor * 100 floors + 20′ = a ‘tower’ height of 1420′. Plugging that into the calculation, the HPBW would touch ground over 3 miles away from the building.

Computers, Software Defined Radio (SDR)

Down the SDR Rabbit Hole

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I’ve always been intrigued by Amateur Radio.  I’m not specifically talking about HAM radio even though that is certainly part of it.

As a boy I traded a telescope for a OLD “world band” radio and spent a lot of time doing stupid stuff like setting the EXACT time on my watch to the WWVB signal.  Honestly, there wasn’t really much to listen to in North West Arkansas in the 70’s on a 50’s era radio, but I tried.  Keep in mind that we didn’t have the internet, so the only way you could find a station was to slowly tune through the bands listening for something that wasn’t static – which meant you had to tune over the station WHILE it was broadcasting (which can be a bit of a trick when only a few stations broadcast 24×7).  Of course you COULD buy a book or subscribe to a magazine that listed some frequencies to try out if you had the money to spend.

Later, I found a copy of an Amateur Radio Relay League (ARRL) book in a thrift store and spent hundreds of hours looking though it, dreaming about how cool it would be to be able to actually do the stuff they were talking about.  Unfortunately, the cost of getting into amateur radio was a deal breaker for me.

In high school I actually went “all in” and bought a CB radio with “significant” antenna and got my CB license (yes, they actually licensed that back in the day).  My call sign was KAKJ4409.

Since then I’ve always found an excuse NOT to get too involved in radio (yeah, I’ve bought a low end Radio Shack scanner, the not so occasional FRS and digital $35 world band receiver at Walmart, etc.  – but I’ve not gotten “into it”).  I’ve considered getting my ham license, but I’ve always found an excuse not to – I didn’t want to learn Morse code, the radios were too expensive, etc.

By this point I bet you are wondering where this is going, well…  I’ve recently gone down the Software Defined Radio (SDR) rabbit hole and I want to document my decent.  For those of you who don’t know, SDR uses computer software to decode a digital version of the radio spectrum.  To do this, you need a device to receive the radio waves and convert them to digital.  Until fairly recently this piece of equipment was incredibly expensive and really only available to the military and research folks, but not any more.

Several years ago, several companies started producing hobby SDR cards.  Then a security researcher figured out that a USB TV tuner that costs about $15 can function as an SDR radio when paired with the right software.  This is the specific rabbit hole I’m headed down.

I’ll post follow up articles as I work my way though the jungle of open source SDR software and I’ll try to leave enough bread crumbs that you can follow my path if you find yourself so inclined.

Happy DX’ing.