Quad Loops
KQ6RH
(C) 1998, 1999, 2000
Ray Jurgens
(Up-Dated 2/25/2000)
Quad loops can be a practical way to get some gain without constructing a full Cubic Quad. The free-space gain of a square loop is about 3.13 dB which is a little improvement over a dipole. If you raise the center of the loop up just one half wave length, a peak gain of 7.75 dBi is reached near 24 degrees elevation. This is not too bad for a simple antenna, and even for 20 meters, the center height will only be about 32 feet. You have the choice of either horizontal or vertical polarization and the decision as to whether to set the spreaders up as a cross or and X. Most people choose horizontal polarization which places the feed at the bottom of the loop. The cross or X selection may depend upon the structure and how close the antenna is to the ground. Normally, the X construction can be more stable in that the weight of the wires is supported by two spreaders while a single spreader must support the entire wire mass as well as part of the weight of the horizontal spreaders in the cross configuration. The cross configuration has definite advantages in height over the X and puts more of the wire at a greater height above the ground. This configuration puts a high current region at the upper apex and gives a better elevation pattern for a given height of center above ground. And if you live in lands of snow and ice, the water runs down the wires and off the corners rather then freezing to the horizontal wires. A definite advantage!
I have constructed quad loops for several bands, but most of my effort went toward a 17 meter loop using 1/2" diameter spreaders extended by 1/4" telescoping sections approximately 2 feet long. There are few secrets to success here, however, the information in the ARRL handbook for driven elements seems to give antennas that are a little small, but that could be due to my environment. The ARRL Antenna book gives the formula 1005/fMhz for computing the length of wire needed for the full loop. It may be better to begin with 1030/fMhz and cut back if the frequency comes out too low. The #14 flex-weave wire is a good choice as it stretches out easily, however, it is definitely over-kill and much lighter wire can be used.
The driving impedance of these loops is a little inconvenient, mine measured about 130 Ohms which agrees well with theory, so a 2:1 balun is recommended or a quarter wave of 75 Ohm line as a matching section. I have found it a bit of a trick to make a good balun that raises the impedance by a factor of two, but there are excellent directions in Jerry Sevick's, W2FMI, book "Building and Using Baluns and Ununs." It is important to make the characteristic impedance of the transmission line winding 100 Ohms as directed, otherwise the high frequency properties will be compromised. An alternative way to feed these loops is to use 300 Ohm twin line or ladder line. I've used a half wave length of 300 Ohm foam filled line with good results by connecting to the balanced line terminals on the antenna tuner. This presents roughly 130 Ohms to the tuner that is expecting 200 Ohms for a perfect match. This small SWR is easily tuned out. And, if weight is an issue, then this is definitely the way to go!
Let's consider the spreader length. Using the formula for the wire length (1005/fMHz), we see that the spreader must be at least (1005/fMHz)/(4*sqrt(2)). For 15 meters, this comes out to about 8.4 feet. Thus, short extensions to the 8' spreaders are required. The extra 0.4' is really no problem for 1/4" rods or tubes, but 17 and 20 meters require special attention. The spreaders for 17 meters must be about 9.82 feet, or an extra 1' 10". In the case of 20 meters, the extensions need to be 4.5'. This latter case is really pressing the limits of what can be done with the light weight materials. All versions of the quad loops require guy lines for wavelengths longer than 15 meters. This means that you must have a boom section that extends about three or four feet either side of the hub. The guy lines can be Nylon fish line, however, Kevlar line works better in this application,because it does not stretch like the Nylon and the lengths can be accurately calculated. Even with guy lines, the 20 meter loop will require lighter wire, i.e., you will not be able to stretch out #14 flex-weave on this structure. Although the 15 meter structure can be managed without guy lines, the spreaders can be bent out to tension the wire. Using #20 stranded copper wire will cost less than 0.25 dB of gain. So you could use stranded hook-up wire. This wire normally comes with insulation, so, you could strip it off exposing the wire to weather, or leave it on and shorten the length by a few percent to compensate for the dielectric loading. I suggest that you leave the insulation on, and buy black Teflon insulation to make the wire less visible. Radio Shack offers a black #18 hook-up wire that also works well.
The construction of loops for 2, 6, 10, 12, and 15 meters is rather straight forward, so the parts list is short. The construction of loops for 17 and 20 meters requires more extensive guy lines, so the parts list grows to cover extra guy ties and spreader extension. We suggest using a short boom with guys for all bands longer in wavelength than 12 meters. As a short boom is required to put the hub in vertical position, adding extra length to the boom and the extra guy lines is really no inconvenience and provides a much more stable structure. Loops for 2 and 6 meters require no guys at all. You should order only enough 1/2" tubing to make the four spreaders for the VHF loops. The 1/2" Guy Ties are convenient for mounting the wire.
In all cases, you will need a mast-to-boom plate with appropriate U-bolts for mounting the 1" boom to the upper mast section. You also must decide if the hub or the boom plate is to be in the center, i.e., both can not be in the center of the boom. In general, it is simpler to put the hub in the center in that all the guy lines can be cut the same. This choice slightly unbalances the the weight at the top of the mast, but this can be compensated easily by adding weight (fishing sinkers) to the shorter side of the boom relative to the boom plate. The construction of boom plates is given in detail in the ARRL Radio Amateur's Handbook. The material of choice is usually aluminum, however, I have been using 1/4" PVC plates with success, and it is even easier to cut and drill than the aluminum and seems to have adequate strength. I have been using four U-Bolts on both the mast and the boom, i.e., two on each side of the mast and boom.
You should order the following parts for 10 and 12 meters:
|
Item |
Quantity |
Description |
|
1 |
1 |
HUB 4-050-100, Central Quad Hub (RFJ) |
|
2 |
4 |
8' 1/2" OD fiberglass tubing, spreaders (MGS) |
|
3 |
1 |
8' 1" OD fiberglass tubing, boom (MGS) |
|
4 |
4 |
GT 4-050 1/2" Guy Ties, for tips of spreaders (RFJ) |
|
5 |
2 |
GT 4-100 1" Guy Ties, for tips of boom (RFJ) |
Additions for 15 meters:
|
Item |
Quantity |
Description |
|
6 |
1 |
1/4" OD fiberglass tubing, spreader extensions, quarter it! (MGS) |
|
7 |
4 |
1/2" stainless steel hose clamps, compression clamps for extensions |
|
8 |
4 |
GT 4-025 1/4" Guy Ties, for tips of extensions or wire attachment (RFJ) |
Additions for 17 meters:
|
Item |
Quantity |
Description |
|
9 |
1 |
8' 1/4" OD fiberglass tubing, spreader extension, halve it! (MGS) |
Additions for 20 meters:
|
Item |
Quantity |
Description |
|
10 |
2 |
8' 1/4" OD fiberglass tubes, spreader extenders (MGS) |
|
11 |
8 |
GT 4-050 1/2" Guy Ties for cross truss ties, (RFJ) |
Note, the spreader extenders can be full 8' lengths of 1/4" tubing, i.e., you don't need to cut them, but you can reduce the weight slightly if you do. They can be cut to 5' lengths leaving 3' pieces for use with smaller quads. The 20 meter structure requires cross truss guy lines (for details see our Construction Page). These require more guy tie points near the centers of the 1/2" spreaders. It is easier to align the tensions if the Guy Ties are separate for the interior and exterior guys, but common tie points can be used if the guy lengths are cut accurately. The total number of guy lines is as follows:
|
Item # |
Quantity |
Approx. Length ft. |
Description |
|
1 |
8 |
9.0 |
interior spreader to boom posts (4' post assumed) |
|
2 |
8 |
13.2 |
exterior spreader extension to boom posts |
|
3 |
4 |
11.3 |
interior perimeter guys (could be wire loop) |
|
4 |
4 |
17.7 |
exterior perimeter guys (could be wire loop) |
|
5 |
8 |
9.0 |
interior cross lateral trusses |
|
6 |
8 |
13.2 |
exterior cross lateral trusses |
|
Table of Guy Line Lengths and Definitions |
for a grand total of 40 lines requiring 80 attachment clips and a total length of material of 1084.4 ft. You should plan a full day for the first construction of the 20 meter loop. Once assembled, the antenna can be disassembled in about the time required to roll up the guy material. Re-assembly takes about an hour.