Using 428MHz Quagi at 446MHz?
|Aug 4th 2011, 12:42|
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A member writes in:
I have a 8 element 70 cm quad antenna that was designed for 428 MHz and has a VSWR of =<1.5:1 at that frequency. I want to use the quad centered on 446.000 MHz +/- 4 MHz at which frequency it presently generates a VSWR of 3.3:1. The antenna is almost flat from 442 to 499.995 with the 3.3:1 VSWR.
Do you know where I can find information telling me how to match the antenna when operating on 446 MHz while lowering the VSWR to =< 1.5:1 measured at the transmitter to coax interface with a power level of 200 watts FM?
I would think that a pi network, T network , or L network would be the solution. I do not have an antenna network analyzer and my only piece of test equipment is a VSWR bridge.
At the 200 watt level, it is impractical to build a conventional matching network. In theory, it could be done, but in practice, adjustable parts can’t handle the power. Chip components might be able to handle the power if you are clever, but the parts your need to handle this sort of power aren’t easy to get. Moreover, you either need to be extremely patient with trial and error, or be relatively skilled with computer modeling to come up with the networks you need—the equations become real messy when one factors in lead lengths at this frequency. At 440MHz, designing conventional networks is like trying to catch a high fly ball in baseball that is being moved around by gusty gale force winds—possible, but even the pros find it hard.
You may be able to match the antenna using shorted stubs—add some T connectors and shorted stubs and experiment until you find a match. In theory, you can match anything with three shorted stubs separated by a quarter wavelength.
But, the best solution is to build an 8 element Quagi designed for FM. Except for the N connector, you can find all the parts you need at the local home improvement store.
Wayne N6NB provides dimensions for a 446 MHz version on this page.
A mistake I’ve seen is to add extra quad spreaders—this hurts the gain significantly at 70 cm. For vertical polarization, the spreaders should be horizontal, not vertical. The loop needs to be fed at the center of the side for vertical polarization.
Zack Lau W1VT
ARRL Senior Laboratory Engineer