Tuesday, August 7, 2012

Simple Antennas for Amateur Radio Operators--a continuing series

KH6JRM's Amateur Radio Blog

Post #181

A small loop antenna for limited space--the MFJ-1788 Super Hi-Q magnetic loop.

I'm a huge fan of loops.  They are quiet and can be hidden if creatively designed.  These antennas can be fed with either coaxial cable, ladder line, or ordinary 300-ohm television twin lead for a simple, effective antenna.  Loops do have their drawbacks.  A full-size 40-meter loop can be large--with a total length of approximately 141 feet.  In my situation, a large full-wave 40-meter loop fits under my house which is raised off the ground by pier and post construction.  A 20-meter full-wave loop is fastened under my garage roof.  Both of these antennas are invisible from the street or from nearby neighbors.  Since these loops are fairly low to the ground, they serve as NVIS (near vertical incident skywave) antennas, which give me strong signals out to 200 to 300 miles.  That range is perfect for afternoon nets and local emergency work.  The 40-meter loop is fed with 450-ohm ladder line and can work all bands from 40 to 10 meters.

But what can you do if a large loop isn't feasible for your situation or your backyard doesn't have sufficient space for a vertical with a suitable radial field?  MFJ Enterprises has manufactured a magnetic loop that can fit on a porch (lanai in Hawaii) or squeezed into a small spot in your backyard.  The MFJ-1788 Super Q magnetic loop antenna is rather expensive and a bit difficult to tune, but it may provide another way for you to get on the air.

In the 07 August 2012 issue of eham.net (http://www.eham.com), there is a review of the MFJ-1788, which can work from 40 to 10 meters.  A similar loop works 30 to 10 meters.  According to the amateurs who have used this loop, the antenna's performance is slightly less than an inverted vee or vertical.  Amateur radio operators reviewing the antenna fault MFJ for several quality control issues, which can be easily corrected with some basic tools.  The overall rating of the MFJ was 4.1 on a 5-point scale.  As N3OV states in the article, the MFJ-1788 is "expensive" ($429.95 USD) but  "not a bad package" for those who need an antenna with a small environmental footprint.  Although the MFJ-1788 is described by N3OV as "a bit of a chore to use" it does produce solid contacts.

Although I prefer homebrew wire antennas, I can see where this MFJ product could be useful in a limited space environment or for emergency use.  You may want to consider this product if your operating circumstances permit no other kind of HF antenna.  In the past, "QST" has published several articles on magnetic loop antennas.  A check in the magazine's on-line index should give you plenty of ideas should you choose to "roll your own."

Aloha es 73 de KH6JRM--BK29jx15