Reasonably scientific way to compare HT antennas - Page 1
Forum section: electronics / rf-microwave

author: djacobow
author_rank: Super Contributor
timestamp: January 21, 2019, 05:06:52 am
content: Ok, so folks are putting HT antennas on antenna analyzers and that strikes me as unlikely to generate very useful assessment of how the antennas really perform. Putting aside that swr isn't performance, your body and the radio are part of the antenna and it'd be tricky at best to create a testing jig that approximates an HT in a human's hands.So, what can you do without a test chamber and lots of fancy gear?I was thinking about going to a large open field, putting various antennas on real HTs and transmitting, with a field strength meter a fixed distance away, perhaps on a ladder, perhaps at different heights.Would that yield useful data?

author: TheSteve
author_rank: Unknown
timestamp: January 21, 2019, 06:08:08 am
content: I figure an analyzer is a good place to start so you at least know the antenna is resonant or close to it. As the user/radio is the counterpoise the SWR actually varies a huge amount depending on how the radio is held and what other objects come near the antenna.On an analyzer I'm sure many antennas are 2:1 or less, but I'm sure many radios see 10:1 or worse when in actual operation.A few of the antennas I've tested give the best SWR when I hold the analyzer in roughly the same manor as I would a radio when in operation, so they were clearly optimized for handheld use.As to determining relative performance without a proper chamber I think you're on the right track. You could use a dipole as your reference receive antenna and take readings at multiple frequencies across the band(s) and at several distances. It will be hard to determine the actual RF pattern each antenna has but it is better then nothing. You will need to simulate the radio being held or try to hold it yourself in a consistent manor for the radio/antenna to have a counterpoise. I would also aim to have a minimum distance between the receiver and transmitter that is many(10+) wavelengths away.

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2019, 06:54:52 am
content: It's comparatively easy with the well priced tools available today.This is how I did it:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

author: cdev
author_rank: Super Contributor
timestamp: January 21, 2019, 03:34:18 pm
content: If you do this in a big grassy field without any metal or trees nearby and site the transmitting antenna and receiving antenna some distance away, do your testing fast enough - or at least take care that the soil isnt just recently rained on and drying, the tests are the same as far as soil moisture especially- then you should be fine.I think some kinds of earth when dryish make excellent natural RF absorbing materials. Wet earth reflects RF to varying degrees, depending on its composition, but thats okay as long as its consistent.(One can actually measure soil moisture very precisely by its ability to reflect GPS signals, in reversed polarity.)

author: djacobow
author_rank: Super Contributor
timestamp: January 21, 2019, 05:35:26 pm
content: Thanks for the replies, folks.I think I will proceed with the open field approach. I have a couple of options nearby. One, a soccer field, should be big enough to have many wavelengths, even at 2m on all sides. However, there is a freeway on one side of the field and the San Francisco Bay on the other, so maybe those are confounders. Another site, a bit inland, it a mostly flat grassy field in a park, surrounded by trees, which can also be a confounding variable.It looks like I'll need an assistant to do this project (someone to read the meter or key the radio) so it might not happen for awhile. I'll report back.

author: cdev
author_rank: Super Contributor
timestamp: January 21, 2019, 05:50:12 pm
content: A valley location sounds perfect. You could use an RTLSDR and record a log of the signal strength (or a 'cfile' of the entire signal) to disk, and just keep a record of what time you tested each antenna, then compare them at home later.