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The length and the width are as long and as wide as they need to be in order to match to 50 ohm, stretching the coil will alter the impedance by a small amount. My shunt coil is longer than it is wide, the reason? simple, the round tube I had to hand wasn't vary wide, it meant I had to add more coils to get the correct impedance, around 8uH as it happens

Shunt feeding a mobile antenna is usually required with a sub 1/4WL antenna, where the efficiency and Rr of the antenna, at resonance, will be low, especially on the lower HF bands.

A less than perfect ground plane is likely to increase loss, and as crazy as it may sound, your feedpoint VSWR may look pretty good as a result, however you're unlikely to notice this with a VSWR meter, that is the domain of the antenna analyser.

<blockquote data-quote="northern35s" data-source="post: 429723" data-attributes="member: 17216">The length and the width are as long and as wide as they need to be in order to match to 50 ohm, stretching the coil will alter the impedance by a small amount. My shunt coil is longer than it is wide, the reason? simple, the round tube I had to hand wasn&#039;t vary wide, it meant I had to add more coils to get the correct impedance, around 8uH as it happens <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=";)" title="Wink&nbsp; &nbsp; ;)"&nbsp; data-smilie="2"data-shortname=";)" />Shunt feeding a mobile antenna is usually required with a sub 1/4WL antenna, where the efficiency and Rr of the antenna, at resonance, will be low, especially on the lower HF bands.A less than perfect ground plane is likely to increase loss, and as crazy as it may sound, your feedpoint VSWR may look pretty good as a result, however you&#039;re unlikely to notice this with a VSWR meter, that is the domain of the antenna analyser.</blockquote>

[QUOTE="northern35s, post: 429723, member: 17216"] The length and the width are as long and as wide as they need to be in order to match to 50 ohm, stretching the coil will alter the impedance by a small amount. My shunt coil is longer than it is wide, the reason? simple, the round tube I had to hand wasn't vary wide, it meant I had to add more coils to get the correct impedance, around 8uH as it happens ;) Shunt feeding a mobile antenna is usually required with a sub 1/4WL antenna, where the efficiency and Rr of the antenna, at resonance, will be low, especially on the lower HF bands. A less than perfect ground plane is likely to increase loss, and as crazy as it may sound, your feedpoint VSWR may look pretty good as a result, however you're unlikely to notice this with a VSWR meter, that is the domain of the antenna analyser. [/QUOTE]

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