Reflected M Beams

KQ6RH

(C) 1998, 1999, 2000

Ray Jurgens

(Up-Dated 2/25/2000)

Reflected M Beams

  The Reflected M beam is made from a pair of wires formed in the letter M that are back to back. One M serves as a driven element and the other can be either a director or a reflector. I consider only the reflector type in this brief discussion. The wires are supported with light weight fiberglass rods that are bent up with guys as shown in the figure below. The main virtue of this antenna is that a 15 meter beam can be built with a turning radius of 7.5 feet and a 10 meter version requires only 5.5 feet. Unlike the VK2ABQ antennas, the Reflected M has a driving impedance that matches rather well to 50 ohms, however, higher gain and better front to back ratios can be obtained with lower feed point impedances. The version presented here has a feed point impedance of 40 ohms, 4.8 dBi gain and nearly 11 dB F/B ratio. Though all my tests have been made using #14 wire, AO indicates that wire as small as #20 can be used without serious degradation. This is not true of the highly optimized designs that approach super-gain performance. These later designs require greater care and more complicated construction techniques, so design details are available in out Products Page.

15 m Reflected M Beam

(Shown here with 8 ft spreaders and seen from the reflector side)

The Reflected M is a planar-type wire beam constructed on a quad support consisting of a hub and four light weight spreaders. We used the 1/2" fiberglass tubes supported by a 1" OD mast extended 3 feet with a 3/4" OD guy post. Alternatively, the 1" mast can be extended through the hub to provide the guy post. Four monofilament fish lines are used as the guys which bend the spreaders up approximately one foot. The antenna is fed at the apex of the M radiator element. The spreaders can be extended by up to 2 feet using 1/4" tubing in order to support a 17 and 20 meter versions. The only disadvantage of this antenna is that it is not easily extended to a multi-band design, however, it is possible to make it dual band by setting second band at 90 degrees to the first. All parts of the dimensions must be scaled linearly for optimum performance, and the wire lengths must be cut accurately (a one inch tolerance is too loose). In this design the driven element and the reflector are of slightly different lengths, and the two are separated by a pair of insulators (we used the light plastic "dog bones" and assumed that the separation was 2 inches and that approximately 2 and 1/2" of wire is needed to tie each to the insulator). If you are unsure about how much wire to leave for the ties, try it first before cutting the wires. Number 14 flex-weave wire works well for this antenna. Although the antenna can be fed directly with 50 Ohm coax, it is better to use a 1:1 balun at the apex. The balun can be supported by a plastic extension added to the quad hub or hung from its center loop if adequate tension is applied at the apex. The distance from the hub center to the apex has been optimized, but this parameter is not as critical as the wire lengths. Pre-measure the locations where the wires attach to the spreaders and mark them with electrical tape or nail polish. Secure the feed point first and then distribute the wire around the structure. You can use 1/2" pipe clamps to hold the wire in place, however, your may want to use some flexible tubing for strain relief at each attachment point. Alternatively, you can use our standard Guy Ties to attach the wire. Pull the reflector apex into its proper place. you will most likely need two nylon or kevlar guys across the front and rear spreaders to keep them from pulling apart as the wire is tensioned. When finished, the apex wires will be pulled away from the fiberglass spreaders except at the attachment point. This separation is necessary to prevent the dielectric material from de-tuning the wires. The typical antenna pattern, F/B ratio, and gain are given in the figure below for a 20 meter version centered at 14.2 MHz. The forward gain is 4.88 dBi and the F/B ratio is better than 10 dB.

This antenna has deep nulls off the sides at 90 degrees to the favored axis.

20 m Reflected M Antenna Pattern
Zin = 40 Ohms

  As mentioned earlier, a 20 meter version of this antenna using the light weight spreaders is difficult to design because the 1/4" spreader extensions bend easily. One solution to this is to use a 8' section of 3/4" fiberglass tubing as the central support for two 8' lengths of 1/2" tubing. This produces an effective spreader of 11 feet. The remaining length (only a few inches) can be spanned by 1/4" solid rod. However, if you are not especially interested in 20 meters, all bands from 10 to 17 meters are easily constructed using the 1/2" fiber glass tubes with 1/4" extensions. An extra set of guy lines is required to hold the extensions in place in the case of the 17 and 20 meter versions.

  The table below gives the design parameters as well as a description of the parameter and equations needed for its calculation. Again, all parameters are scaled relative to the length of the attachment point to the spreader, so these points should be marked with a dab of nail polish or tape. I attached the wire directly to the fiberglass spreader with a half inch hose clamp and used Teflon tubing for strain relief. The spreaders were pulled up with guys to an extender on the central mast as shown in the picture. Don't forget to put the sleeves on the wires first and also mark the locations of the bends with nail polish.

  The Reflected M design has only three design parameters to be optimized. Since the entire antenna is to fit in a perfect square, only the size of the antenna, the location of the feed point from the center, and the location of the insulators as a proportion of the sides is optimized. (It is possible to modify the length of the insulator, but this leads larger spreader lengths). In this design, I consider using the parasitic element as reflector, however, it is possible to make it a director.

Parameter	Description	Equation	Value for 28.5 MHz
Spreader	Attachment point to spreader	152.93/fMHz	5.366 feet
Side	Length of a Side to attachments	sqrt(2) * Spreader	7.589 feet
x	Semi-Side length	Side/2	3.794 feet
a%	Location of apex from center in %	7.63%	(design parameter)
a	Length of apex from center	a%/100 * Spreader	0.409 feet
Ls	Length of radials from apex to attachments	sqrt[x^2 + (x-a)^2]	5.085 feet
ins	Insulator Spacing	(dog bone)	0.167 feet
d%	Driven element side in percent	47.97%	(design parameter)
r%	Reflector element side in percent	100 - d%	52.03%
Ld	Length of driven element side	d%/100 * Side - ins/2	3.552 feet
Lr	Length of reflector element side	r%/100 * Side - ins/2	3.865 feet

Design Parameters for the Reflected M Antenna

  Note that the length of the driven elements apex wires is Ls + Ld plus anything you need for attachment to the insulators. The length of the full reflector element is 2*(Ls + Lr) plus anything you need for two attachments to the insulators.

  This antenna exhibits a low SWR over the normal amateur bands but its performance will change as with frequency. In order to force the optimization to maintain a wide bandwidth , the optimization has been carried out simultaneously at three frequencies that span approximately 200 kHz. This procedure assures that bandwidth covers a substantial portion of the amateur band in all cases.

  The Spreader length has been adjusted to account for the difference between the theoretical and the actual lengths required. This difference is most likely due to the proximity of the wire to the fiberglass spreaders and the excess capacity at the tie points of the insulators. This version of the Reflected M antenna should have a driving impedance of about 40 Ohms at the design frequency. The actual properties may be influenced by the environment, so you may need to adjust the lengths slightly. Adjustments should be done by re-scaling all dimensions, i.e., modify the scaling factor for the spreader length at the top of the table and recalculate all lengths. A good test of having a properly working version is getting the driving impedance correct at the design frequency. You can make some adjustments by clipping alligator clips to the wire at the insulators. These add a slight bit of capacity and lower the frequency. This is helpful in determining if the driven element or the reflector element needs to be adjusted. Normally, adding the clips to the reflector will move it farther from the frequency of the driven element reducing the coupling and thus raising the impedance. So if the impedance comes out low, you can use the clips to determine which element to work on. If the antenna is going to be temporary, you can leave the clips on and avoid trimming the wire. if the frequency is too low, you'll have to trim wire. If you adjust this antenna for a perfect 50 Ohm match, you will sacrifice front to back ratio and a slight bit of gain. On the positive side, you gain a little bandwidth.

  If you are interested in more gain and a higher F/B ratio, it is possible to get the performance shown in the figure below by lowering the driving point impedance to 25 Ohms. A design sheet for this antenna is available from our products listing. This antenna requires a matching balun for broad band performance, however, the added gain and much improved front to back ratio are well worth the extra effort and slight increase in cost. As you can't have everything for nothing, this design has a narrower bandwidth than the 40 Ohm design and requires very careful attention to the exact lengths and to the accuracy and symmetry of the fiberglass structure. More guy lines are required to achieve the required rigidity of the structure and its ability to properly tension the wire. Note, this antenna is only slightly poorer in performance than a full size two element cubic quad but is much smaller and weighs about 1/3 as much.

20 m Reflected M Antenna Pattern
Zin = 25 Ohms

  The parts required to build the Reflected M wire beam depend upon the the band desired. 10, 12, and 15 meter bands require spreaders less than 8' feet and no spreader extensions are needed. The 17 meter band requires short extensions of less than a foot including the overlap. Reaching the 20 meter band requires extensions of about 3' or more including overlap. Also required is a much more complicated guying procedure, however, if 20 meters is your goal, it can be reached with the light weight fiberglass structure. With respect to the parts quantities listed below, for 17m you will need additionally one 8' length of 1/4" fiberglass tubing which should be cut into four equal parts. For 20 meter construction, you will need two 8' lengths of 1/4" tubing which should cut in half to make four extenders. The extenders require stainless steel hose clamps to compress the silted ends of the 1/2" spreaders so as to lock the extenders in place. The number of Guy Ties you need depends upon if you plan to use these to attach the wire. Guy Ties provide a very clean installation that makes the antenna easy to assemble and disassemble. Alternatively, you can use hose clamps to attach the wire directly to the fiberglass tubing. The 20 meter version of this antenna requires cross lateral guys at two locations. These should be independently adjustable to accommodate the division of loading among the guy lines (see Construction Details section). We also highly recommend using Kevlar guy lines rather than Nylon fishing line for the 20 meter version. The 1/8" diameter black Dacron can be used with greater visual impact.

All parts to construct the Reflected M wire beam are available through MGS. In all cases, you will need the following parts:

Item	Quantity	Description
1	1	HUB 4-050-100 Central Hub for 1/2" tubing an 1" mast (RFJ)
2	4	8' 1/2" OD fiberglass tubing (MGS)
3	1	8' 1" OD fiberglass tube (MGS)
4	1	GT 4-100 1" Guy Tie (RFJ)
5	4	GT 4-050 1/2" Guy Tie (RFJ)
6	4	GT 4-050 (as above) (optional for wire supports) (RFJ)

 Extension to 17 meters, (add)

Item	Quantity	Description
7	1	8' 1/4" OD fiberglass tubing (MGS)
8	4	1/2" stainless steel hose clamps (hardware)
9	4	GT 4-025 1/4" Guy Ties (optional for wire supports) (RFJ)

Extension to 20 meters, all of the above except the optional wire supports, (add)

Item	Quantity	Description
10	1	8' 1/4" OD fiberglass tubing (MGS)
11	4	GT 4-050 interior cross lateral guy ties (RFJ)
12	4	GT 4-050 exterior cross lateral guy ties (RFJ)
13	4	GT 4-025 exterior cross lateral tip guy ties (RFJ)
14	4	GT 4-025 (optional wire supports)

The computation of the amount of wire required is discussed above. The amount of guy material can also be computed for each band. This is discussed more fully in the Construction Details section. However, because the 20 meter version of this antenna requires more guy lines than the others, the number and approximate lengths of each is given in the table below.

Item	Quantity	Approx. Length ft.	Description
1	4	9.0	interior tip to guy post, assuming 4' post
2	4	11.5	exterior tip to guy post
3	4	11.3	interior perimeter
4	4	15.3	exterior perimeter
5	8	9.0	interior lateral trusses
6	8	11.5	exterior lateral trusses

 The lengths given above are maximum lengths for estimating the amount of guy material required and are not those actually required to properly mold the antenna. A total of 32 guy lines with 64 guy clips and 352 ft of guy material are required for the construction of the 20 meter Reflected M beam.

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