# Portable radio am/fm question

**URL:** <https://boards.straightdope.com/t/portable-radio-am-fm-question/124683>\
**Category:** Factual Questions\
**Created:** [August 20, 2002, 1:31am UTC](https://boards.straightdope.com/t/portable-radio-am-fm-question/124683 "2002-08-20T01:31:26Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Cat\_Patrol](https://avatars.discourse-cdn.com/v4/letter/c/dbc845/32.png) [@Cat\_Patrol](https://boards.straightdope.com/u/Cat_Patrol)\
**Post date:** [August 20, 2002, 1:31am UTC](https://boards.straightdope.com/t/portable-radio-am-fm-question/124683/1 "2002-08-20T01:31:26Z")

</div>

Why do portable radios have the AM antenna on the inside of the radio, instead of on the outside like the FM antenna?

---

<div class="post-metadata">

**Author:** ![n5tp](https://avatars.discourse-cdn.com/v4/letter/n/71c47a/32.png) [@n5tp](https://boards.straightdope.com/u/n5tp)\
**Post date:** [August 20, 2002, 1:44am UTC](https://boards.straightdope.com/t/portable-radio-am-fm-question/124683/2 "2002-08-20T01:44:11Z")

</div>

The usual method of construction for an AM antenna is to wind small wire around a powdered-iron rod (also called a ferrite rod). Otherwise, the antenna would be too long for practical purposes. The ferrite rod is fragile, and is mounted inside the case for it’s own protection.

---

<div class="post-metadata">

**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\
**Post date:** [August 20, 2002, 2:20am UTC](https://boards.straightdope.com/t/portable-radio-am-fm-question/124683/3 "2002-08-20T02:20:06Z")

</div>

The AM antenna is of such construction as described by **n5tp** because of the much lower band of frequencies for which it needs to be receptive. AM broadcast is 535KHz-1605KHz, compared to the FM band at 88MHz-108MHz. Now look at the formula for resonant frequency of an LC (inductve-capacitive) circuit, in which the AM antenna coil of wire acts as L:

ƒ[sub]o[/sub] = (2×pi×/LC)[sup]-1[/sup]

That’s supposed to be the square root of LC but the symbol font seems to be hosed up for some reason. You can see that the values of L and C must be larger at lower frequencies. A coil of wire wrapped around a ferrite core has much greater inductance than a straight telescoping rod, and therefore works better at lower frequencies.