active FM / AM loop antennas  teardown - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2019, 03:19:53 am
content: so if you have a stereo and you searched for commerical antennas, you undoubtly found some active loop ones that claim to increase reception.What I found interesting was a 'gamma loop'https://www.amazon.com/Terk-Amplified-Indoor-Antenna-PIB/dp/B00008V3EEWhat do they mean by this? Something with a gamma match ?The only thing I found is thishttps://www.nytimes.com/1989/05/28/arts/sound-technology-trims-the-rabbit-ear.htmlIs it unique to Terk? I take it the advertizing is bullshit, and they mean the antenna on amazon is actually a reduced size loop with a amplifier (where the size reduction makes it worse then a full size antenna connected to the same amp)?

author: @rt
author_rank: Super Contributor
timestamp: August 04, 2019, 08:01:03 am
content: They might mean a “Halo”. A magnetic loop with gamma match, that would be good for SNR, then amplified.It looks like they could fit one for FM broadcast in there.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2020, 04:19:23 pm
content: I got this antenna today. It seems to do something, or at least the amplifier at least attenuates, because if I do a FM sweep with the dial changed, the reception changes on FM. It is also directional, but I am working on a mounting system near the stereo set because its hanging off a cabinet door ATM. Also the line does appear to make it more noisy, not sure if its from unnecessary amplification or line noise yet. Need to power it from a battery pack.Maybe I will set it up on a motorized antenna positioner on a tripod in the middle of the room and do some more precise testing with batteries before I invest any more into integrating it into the furniture nicely. I should get data during night and day and in rain and snow if possible this year. I need to build a plastic pedestal to offset antenna from the metallic tripod and antenna positioner first by some decent distance. Should be a useful fixture.Physically the antenna will benefit from a extension on the gain adjust knob, it looks nice and perhaps is appropriate if tuned to a serious listener, but for testing it needs to have a bigger protruding knob put on it, perhaps one that fits over on a snug fit with a good finger hold, it is very recessed. More like a trim adjustment then something for frequent scanning, but this begs the question as to why bother with this system at all.. oh well lets just call it a living room relaxing fun activity rather then research when the system is established. It is meant to be adjusted to your favorite radio station and left alone.Another slightly criticism was that the AM leads came with solder on them, so I replaced that with 26AWG ferrules. I don't want solder going into the clamp terminals, even if its non issue in a 300 ohm low power system on cheap equipment. It is probobly fine for home entertainment.

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 03:16:09 pm
content: well it defiantly works, when I carefully scan it on my new positoner I can pick up certain radio stations at certain angles with certain amounts of gain.

author: madires
author_rank: Super Contributor
timestamp: January 03, 2021, 04:11:06 pm
content: Yep, you're experiencing the pros and cons of directionality.

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 04:46:40 pm
content: well i mean the gamma match thing actually works with whatever its doing, i thought it might have been a scam. its actually a loop antenna or something under that plastic

author: madires
author_rank: Super Contributor
timestamp: January 03, 2021, 06:47:33 pm
content: Old tube radios often have a simple loop antenna for AM on the inside of the back cover. It's nothing special. And there are also magnetic loop antennas with a pickup loop and a capacitor for tuning. However, I'm wary of RF amplifiers because the cheap ones can add a lot of noise and make things worse.

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 07:11:57 pm
content: this amplifier does make it worse in some cases but there are signals which are not picked up if the gain is turned down, so its actually an amplifier of some sort. I thought the whole thing might have been a scam. When I figure out the voltage tolerance I will try to power it from batteries, I hope I do not need a regulator circuit.Kind of wonder how well the one with the preselector works (passive).Not sure on the gamma match yet but at least they got the electronics right, I am kind of impressed it does somethingDoes anyone have a standard common FM antenna model or design that I can compare this terk to?not sure what form the FM antenna is, if it is a loop, because its under the black disk, I will refrain from tearing it down until I do some comparative test.I am looking for a standard size antenna, since this thing seems to want to be the king of the coffee table size category. Let's see how well it does compared to some gargoyle.

author: coppercone2
author_rank: Super Contributor
timestamp: January 07, 2021, 11:33:44 pm
content: the wall wart that came with this thing is good, on a DMM it reads lower then a adjustable low noise power supply (both without load). Should scope it to 20MHz. Measures the same as a 12V NiMH battery pack on a 34401A AC mode.Will hook both up to jim williams 100KHz low noise amplifier with the 4th order filter then do RF noise tests.

author: coppercone2
author_rank: Super Contributor
timestamp: January 15, 2021, 07:42:50 pm
content: Oh, it looks like preamp I was using does not like high voltages. My low noise lab supply reads something like80uVRMS close to specified (well measured with 100KHz bandwidth! they might measure with 20MHz), a 5V battery pack reads ~1.5 uV RMS (NiMH D cells), but the preamp drops to 0 output when you raise the DC bias close to the rail, so I was just unable to measure the wall wart correctly.