Modifying a standard cheap whip antenna equipped shortwave receiver - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: February 07, 2019, 06:48:30 pm
content: Got a little 5$ whip antenna equipped shortwave receiver laying around me. Just out of curiosity wanna know whether there is any way to modify that thing to accept a standard 75 ohm coax connecter. For me that whip antenna is preety much useless as it doesn't give a good signal strength even for the local SW stations. Adding an outdoor antenna would be quite a nice experment for me. But found no clue on hot to add an external antenna input. Anybody there who has tried this?

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 07, 2019, 09:58:54 pm
content: I'd just get a 75 ohm socket and solder a short lead with alligator clip to the telescopic antenna.Although a few radios have a 3.5mm antenna connection.Connecting to the earth is harder but you may be able to bring the negative battery terminal out somewhere.Or if you want to be a butcher, open up the radio, remove the whip and have a couple of wires coming out to a small socket or similar.

author: Dom13c
author_rank: Regular Contributor
timestamp: February 08, 2019, 08:32:37 am
content: Is it safe to assume that the impedence between the whip antenna and the ground of the radio circuit would match the 75 ohm impedence presented by the coax cable. I'm worried signal attenuation would take place if impedance mismatches.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 10, 2019, 01:33:12 am
content: Quote from: Dom13c on February 08, 2019, 08:32:37 amEfficient power transfer through good impedance matching is important for transmitting but less so for receiving.This is because modern receivers are quite sensitive (especially those designed for use with small telescopic antennas) and the typically high noise of urban environments.It's more important to go after signal / noise ratio rather than absolute signal strength so loss due to a mismatch is unlikely to have much of an impact on redability.It you really did want good impedance matching you could build a small antenna coupler and have it between the antenna and the receiver.Even better if it is a type of coupler with a selective tuned circuit as it would reduce any interference caused by poor image rejection (quite possible on cheap receivers).

author: babysitter
author_rank: Frequent Contributor
timestamp: February 10, 2019, 09:07:33 pm
content: Are you certain its using the whip antenna, not a internal ferrite bar antenna? Easy check, direction sensitivity.Then you could just couple to a longer wire - a method my 5 year old (re)discovered after he watched me check a chinese class D audio amp for annoying emissions with a TH-F7E. Proof:

author: cdev
author_rank: Super Contributor
timestamp: February 14, 2019, 05:30:18 am
content: The best HF antenna you could ever hope to find for a small space would be the one that vk3ye is holding in his hand in his profile pic. It doesnt have to be round squre is fine, but you will need a variable capacitor to tune it.You can add a coax connector to a small shortwave radio and the impedance shouldnt matter much as you usually have ample signal especially with a magnetic loop. Can you locate the ground on the PCB internally? Its usually the largest area of copper - if in doubt, start with teh negative of the battery. But make sure you dont short anything out. My little SW radio has the outside of the power jack as POSITIVE.

author: Dom13c
author_rank: Regular Contributor
timestamp: February 14, 2019, 09:06:28 am
content: Quote from: babysitter on February 10, 2019, 09:07:33 pmYup, its actually a ferrite bar antenna as you said.😂 I misunderstood the wiring.

author: Dom13c
author_rank: Regular Contributor
timestamp: February 14, 2019, 09:13:10 am
content: Quote from: cdev on February 14, 2019, 05:30:18 amI've actually opened the radio case quite a few times and it was not at all a problem to find the ground. And yes, I'll give the magnetic loop antenna a try . Had a thought of constructing a dipole but it's just too much space consuming ,as you say.

author: babysitter
author_rank: Frequent Contributor
timestamp: February 14, 2019, 10:50:15 am
content: Great, then just hold it up to longish metal parts, or wrap wire around the case of the receiver. e.g. mains cable of other devices. YMMV.