13 .pdf


Aperçu du fichier PDF 13.pdf - page 51/52

Page 1 ... 49 50 5152



Aperçu du document


Fig 13-79—Arrangements for feeding the
driven loop of the 40-meter inverted-delta
quad, as a vertical on both 80 and
160 meters. Two LC networks are used. The
values indicated are approximate.

4.6. Quad or Yagi
I must admit I have very little first-hand experience with
quad antennas. But I can think of a few disadvantages of quad
antennas as compared to Yagi antennas:
• Much better F/B can be obtained with Yagis.
• Quads are three-dimensional; you can’t easily assemble a
quad on the ground, and then pick it up with a crane and put
it on the tower. You must do a lot of assembly work with
the boom in the air.
• Wires break.
• It is a non-efficient material user—All the metalwork you
put up is not part of the antenna; it is just a support
structure.
• So far as electrical performance is concerned, a well-tuned
quad antenna should marginally outperform a Yagi with
the same boom length, at least as far as gain is concerned.
The difference, being in order of a fraction of a dB to
maximum of 1 dB, is more of an academic than of a
practical nature.
To prevent ice build up on the quad wire, you can feed a
current (ac or dc) through the loops. The voltage should be
adjusted so as to raise the temperature in the wire just enough
to prevent ice loading.
The fact is that the great majority of rotatable arrays on
the low bands are Yagis. This seems to indicate that the
mechanical issues are probably harder to solve with quads
than with Yagis.

5. BUYING A COMMERCIAL LOW-BAND
YAGI ANTENNA
5.1. 80-Meter Yagis
5.1.1. The linear-loading approach
To my knowledge, there is no manufacturer who is

currently offering a full-size 80-meter rotatable Yagi antenna.
The 2 and 3-element shortened Yagis with linear-loaded
elements, originally developed by Mike Staal, K6MYC, for
KLM more than 20 years ago, have held up over the years.
Later M-Square, Mike Staal’s new company, and Force-12
sell 80-meter Yagis using linear-loaded Yagis based on his
original design. The merits of the linear-loaded design have
long been established. The only inherent design compromise
seems to be the sacrifice in top-notch directivity, which is
caused by some radiation from the slant loading wires (see
Section 3.7).
5.1.2. Mechanical issues
When deciding to buy one of these antennas, check the
mechanical issues closely. This is what makes an 80-meter
Yagi last or not. I must admit I am really scared when I see how
some of these antennas are made. I see 80-meter Yagis using
booms with a wall thickness that is less than half of the wall
on my 40-meter Yagi boom! I see how a simple aluminum
plate of a few mm thickness, is connected by a few simple
rivets to the boom, and this is supposed to hold the 25-meter
long element in a lasting way. I have doubts these antennas can
ever stay up in windy areas. In my QTH they would not last
one winter!
Mechanical issues are the real issues for a long-lasting
80-meter antenna. So, if you decide to spend a lot of money,
take a very close look at the mechanics. An antenna built to
withstand high winds and lots of ice loading will inevitably
use more aluminum than a flimsy antenna that won’t with­
stand a 90-km/h breeze. And aluminum costs money. There
is a price for a good mechanical design. There are no two
ways about it.
I know it takes approximately 45 kg of 6061-T6 alumi­
num to make a full-size 40-meter element that will withstand
Yagis and Quads

Chapter 13.pmd

51

2/17/2005, 2:50 PM

13-51


Ce fichier a été mis en ligne par un utilisateur du site. Identifiant unique du document: 00236958.
⚠️  Signaler un contenu illicite
Pour plus d'informations sur notre politique de lutte contre la diffusion illicite de contenus protégés par droit d'auteur, consultez notre page dédiée.