How long does my AM radio receiver antenna wire need to be?


Joined Mar 24, 2022
I'm looking for the design formulae for creating a LW/MW/SW (and AM Commercial) Broadcast antenna like the ones that used to be installed in the old RCA/Silvertone/etc tube radios, usually wound in a flat square on the fibre-board back cover of the radio itself.
My plan is to place it on such a backing and affix it to the back of a piece of wall art as a stealth antenna.
One on one wall with E/W directionality, and one on another wall with N/S directionality, switching between the two with a selector at the receiver.
My goals are to wind enough wire to provide a good RF capture surface, but also to have the wire coil show an actual impedance of near 52Ω at the feedpoint to match the antenna input of my General Coverage 100Hz to 30Mhz Receiver, with as much of the signal amplitude transferred through 50Ω unbalanced coax, as possible.
I don't mind placing a small air-variable cap at the antenna for tuning the loop, as long as the array's impedance can be maintained.

Icing on the cake would be if the antenna were able to handle a small amount of RF transmitted through it (perhaps 5-10 watts max) and be easily matched with a minimal L/C tuning circuit. Another reason for desiring the natural impedance match.

Thoughts ?
Thanks !

(sorry, 10 min edit limit expired in the process of editing)


Joined Aug 27, 2009
I'm looking for that design formulae too. ;) There are active mag-loop antennas for wide-band receive but for transmit you are likely in magical thinking territory.


Joined Mar 19, 2019
@TwinSteel And Welcome to AAC!

Just about any metal object can be tuned as an antenna. When antenna tuners first became popular the infamous lawn chair antenna became in vogue.

sparky 1

Joined Nov 3, 2018
The subject how long should an AM radio antenna be. The conductive wire is part of a resonant circuit.
The other elements in a resonant circuit can affect the resonance of the conductive wire which is called the
electrical length. Three main ideas are 1. The resonant circuit (including the antenna). 2. The ground. 3. The pre-amplifier.

When a long wire antenna is made resonant, we might say the antenna has14 dB gain. Let's say 14dB still cannot receive a weak signal.
We might build a pre-amplifier for medium wave, it has 16dB gain so in total there is 30 dB gain.
Most modern radios have preamplifiers that are usually better than really old radios.
A 30 dB gain would help identify if reception is possible, sometimes it is a struggle between signal and noise.

A signal generator and the FFT (or RTL-SDR (software defined radio)) could be used to evaluate the gain of the antenna and preamplifier.
The signal generator would need to be strong enough to be measured before a specific attenuation is made.
By using signal strength logger data, the AM stations signal strength and your antenna data can be analyzed more objectively.
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Joined Aug 21, 2017
Rule No1 - never permit wire longer than lambda quarter. Shorter it may be Rule No2 - as longer wite as better signal strength; as nearer to lambda quarter as Z is nearer to 75 Ohms and have less reactance. When the WSVR indeed is important (read, You are going to use it as transmitting antenna) then just let measure it! Bad news - for so low frequencies my most beloved PS200 is too fast, but little bit more quirk-around NanoVNA v2 and even v1 are just ideally suited. Teen of bucks and You are a Human :) . Its very difficult to not loose a face if You cannot see what You are doing and what You are getting, isnt it??