Antenna Adapter - Page 1
Forum section: electronics / beginners

author: JSanders
author_rank: Contributor
timestamp: October 01, 2023, 12:31:14 am
content: Wasn’t sure if I should ask this here or in the RF section. Get really bad FM radio reception on my cell phone inside my house. In fact get next to nothing without plugging an antenna into the headphone jack (see picture).Have a big old antenna in the attic able to get a whole slew of digital TV stations with pretty amazing picture quality. Figured I should be able to slap a splitter on the coax & wire up my own headphone jack adapter for my cell phone but didn’t work. The FM reception is really bad, almost worse then using nothing at all.Am I wrong in thinking FM signals would be picked up through my attic TV antenna?Thanks.Edit: Tried two different splitters I had, both 2-way. First 5-1002MHz –3.5dB, Second 5-1200MHz 3.7dB

author: Ian.M
author_rank: Super Contributor
timestamp: October 01, 2023, 03:32:24 am
content: The bottom of the DVB TV broadcast band is 1.6 times the frequency of the top of the FM broadcast band. It is therefore very unlikely that any directional TV antenna will have useful reception in the FM band. Back in the day, it was common practice to fit an often omnidirectional FM antenna to the same mast as a TV antenna, and combine them with a diplexer to let them share the same downlead and distribution amps. Also the phone almost cetainly only has a single antenna connection, using the shell of the headphone jack, relying on the phone chassis as the counterpoise for an approximate quaterwave antenna formed by the headphone cord, so connecting it to coax without a matching network and a ground connection (e.g. via the charging port) isn't going to do much.https://www.digikey.com/en/blog/getting-headphone-leads-to-do-double-duty

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 02:23:09 pm
content: So the antenna pictured has a tip/ring/sleeve connection and is a 'single ended monopole' antenna. Your attic antenna has Two conductors. How did you connect it to your cellphone? Personally, I am unaware of cellphones with built in FM VHF reception? Guessing one of your antenna wires would go to the sleeve connection (ground) and one wire probably to the tip (hot) connection. I don't know what your antenna downlead is. If it is coax you can probably wire it correctly and directly to your headphone male connector. If it is balanced twinlead you will need a 300 ohm to 75 ohm balun. I should think even a DVB antenna wired correctly would work better than the little monopole antenna in your picture. When it comes to antennas, bigger is better and resonance is best!! Maybe tie the shield and the center conductor of the coax together and connect to the 'tip' of the T.R.S. connector? Then the entire run of wire including the attic antenna would become a 'longwire' antenna. Tell us what you find.

author: JSanders
author_rank: Contributor
timestamp: October 01, 2023, 02:46:03 pm
content: Ian.M, thanks for the DigiKey link, great article. Was doing really well reading your post until I got to the “counterpoise for an approximate quaterwave” when things went right over my head.As the DigiKey article mentioned, my ‘unshielded’ wired headphones also work as a FM antenna, just not as good as the antenna pictured in my first post. Kind of assumed connecting to my attic antenna would be like using really long headphone wiresPretty much lost when it comes to anything RF and Frequency Modulation is like black magic to me. The antenna I have in the attic (see picture) currently receives channels in the range of 2 - 66. I thought that the FM band was hiding out in the lower channel range, 7 and lower of VHF broadcasts. Oh and as a precaution I bypassed the TV amplifier in case it has a FM trap, still nothing.CaptDon, Connected the coax signal wire (center lead) to the headphone ground thinking it would make one really big monopole antenna. Wasn’t bold enough to start playing around with other configurations worried that RF voltage could possibly hurt my phone if I tired to hooking up to either the tip or ring of the TRS connector.Edit: The Phone is a Moto G Play 2021

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 05:11:58 pm
content: Hooking the coax to the ground (shield) I would surely expect to accomplish nothing. I wonder which pin T, R or S that mono pole antenna is connected to? I would doubt it is connected to the shield but perhaps it is. No telling how they couple to the FM receiver internally.

author: JSanders
author_rank: Contributor
timestamp: October 01, 2023, 05:25:36 pm
content: When first tried this kind of figured it should probably be the ground and the DigiKey schematic supports that as well – if I’m reading it correctly. Also checked things with the TRS connector & my meter, got a direct connection from the ‘sleeve’ to the metal antenna.

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 06:47:53 pm
content: Ahh, O.K., I see that the inductor allows the RF to not be shorted to a real (chassis) ground within the cell phone but still allows a return for audio frequencies of the headphone elements. It looks like the ferrite beads decouple the RF also. Not really sure why you don't get better reception with various iterations of hooking the coax center conductor or shield to the sleeve? One would think the coax itself would be acting as a long wire antenna and provide some gain over just the headset or the monopole antenna. Strange indeed. I could see no net gain if the attic antenna is somehow grounded at the antenna end. Sadly, I have not seen a cell phone with built in FM receiver. Don't have any experience with them.

author: nali
author_rank: Frequent Contributor
timestamp: October 01, 2023, 07:16:26 pm
content: Assuming your phone has smarts, have you looked at the Radio Garden app? It basically lets you twawl the globe to play local radio stations and has a suprising amount of provincial stations included.

author: JSanders
author_rank: Contributor
timestamp: October 01, 2023, 07:55:37 pm
content: Nali, grabbed Radio Garden to give it a try but doesn’t have my local stations. What prompted all of this is about a week ago lost all power, even cell towers went down, the radio was all I had left.Been a while since I’ve been in the attic where the antenna lives. Its jammed into & above the vaulted ceiling of the master bedroom, really a tough crawl through a lot of fiberglass, now wondering if maybe the antenna itself has an FM trap. Been reading where some antennas have this feature although appears to be rare.

author: JSanders
author_rank: Contributor
timestamp: October 08, 2023, 09:53:58 pm
content: Waited for cooler temps to climb in the attic where I found an old RadioShack FM Trap on the coax. Ditched the trap, soldered the coax to the sleeve / ground of the TRS connector and everything’s working great. And TV signals don’t seem any different without the trap.