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300mw vs 5 watt

638RS

638RS

Well Known Member
I just finished my annual and went for a test flight this morning. During the annual I installed an inexpensive second com antenna on the belly to a BNC connector in the cockpit to use as a emergency back up with my Aviation handheld and with my APRS transmitter.

I thought I would take the occasion to compare two APRS transmitter units of different power. One is a microTrac putting out 300mw running off a nine volt battery, the second is a Yaesu handheld with a Byonics encoder putting out 5 watts. Both were used with a Byonics puck GPS

The flt from KHIO (Hillsboro) to 4S9 (Mulino) was with the 300-mw unit. The flight back from Mulino to Hillsboro was with the 5-watt unit. As you can see the 5-watt output is more reliable but the 300 mw unit is certainly adequate. Both units had hits on the ground; Prior to this flt I was using a wing tip installation with a twin lead dipole antenna and never saw a hit below 600 ft. As others have said, the antenna is the important element in the system.

Here is a link to the track

http://www.mail2600.com/cgi-bin/track.cgi?call=N623RS&last=1
 
300mW vs 5W is about 12dB, a significant amount. However, the difference between a good antenna with a proper ground plane and proper polarization and a poor one can easily exceed this amount.

If you have a good antenna, 300mW should be plenty - you normally will have a good line-of-sight, even in western Oregon.

TODR
 
638RS said:
I just finished my annual and went for a test flight this morning. During the annual I installed an inexpensive second com antenna on the belly to a BNC connector in the cockpit to use as a emergency back up with my Aviation handheld and with my APRS transmitter.

I thought I would take the occasion to compare two APRS transmitter units of different power. One is a microTrac putting out 300mw running off a nine volt battery, the second is a Yaesu handheld with a Byonics encoder putting out 5 watts. Both were used with a Byonics puck GPS

The flt from KHIO (Hillsboro) to 4S9 (Mulino) was with the 300-mw unit. The flight back from Mulino to Hillsboro was with the 5-watt unit. As you can see the 5-watt output is more reliable but the 300 mw unit is certainly adequate. Both units had hits on the ground; Prior to this flt I was using a wing tip installation with a twin lead dipole antenna and never saw a hit below 600 ft. As others have said, the antenna is the important element in the system.

Here is a link to the track

http://www.mail2600.com/cgi-bin/track.cgi?call=N623RS&last=1
Click to expand...

Your tracks look good and this is a valuable data point for those wondering how 300mw compares to 5w. Thanks for making the effort to document it for us.

The reluctance of some RVer's to install another external antenna is understandable, but in my opinion if the pilot is serious about optimizing APRS tracking, the external antenna is easily justified. Having the antenna available for use with a handheld com is a bonus.
 
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The antenna is a simple stainless steel bent whip antenna purchased from Wag Aero. It was left as purchased and not modified for the ham band. I thought about shortening it for the ham band but after testing I am just going to leave it as is...
 
1/4 wave antenna length

The 1/4 wave whip length formula is 234/f = length in feet, where f is the freq in mhz. Simply put, about 19 1/2" at 144.000. If your antenna is not 1/4 wave, you will have a substantial xmtr to antenna matching problem, losing much of your 300mw, and you will likely have multiple lobes/nulls in your antenna pattern. The exception would be a 5/8 wave antenna, but such is not suitable for aircraft use. SWR analyzers are relatively inexpensive by airplane parts standards, but not really required for APRS. A common SWR analyzer is the MFJ-259B, does HF thu 170mhz, so can also check airband antennas, etc..
 
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