Today we present another article inour non-fiction writing contest – by PrepperDoc
This is a fun, almost novice-level construction project for anyone who has a 2-meter ham radio walkie talkie (e.g., the $26 Baofeng UV-5) and would like more effective range without having to buy an amplifier. If you are currently using a short rubber-duckie antenna, this high-gain yagi antenna may well make your radio appear to have 6-10 times its power on 2 meters, doubling or tripling your range in relatively open areas, for a total cost about $21. [Dense high-rise central business city centers may not see as great an improvement, nor areas with impenetrable hills.] That may just make it possible for you to reach useful repeaters or friends from inside your house.
Using thick multi-ply packing/shipping corrugated cardboard to simplify construction (see the accompanying photo of the completed antenna), this antenna is for indoor or very protected outdoor usage only; it has to stay dry. This antenna can be “hung” like a large painting on any wall that “points” it in the right direction, as long as there isn’t a lot of metal in the wall, and the house doesn’t have metal siding (or buried rebars).
It can even be covered up with a large poster or a world map, and no one will even know it is there! This project can help teach newer users quite a bit about radio antennas. I came up with the idea after building several simple outdoor 2m/70cm yagi-style beam antennas (using a 1″x2″ wooden boom for support) and realized it could be adapted for indoor use. While my discussion is for the common vertical polarization (elements are aligned vertically) on 2 meters, it could also be set horizontally if your counterparty is using horizontal polarization.
It will also work on the 70cm band, but my results there are less satisfying.
This antenna was designed for center frequency of 146 MHz and 440 MHz. On 2 meters, this antenna will give you roughly 6dB gain over a full size dipole (6 “dBd”) , and probably 12 dB over the stubby “rubber duckie” antenna that probably accompanied your handheld transceiver. This is because electrically “very-short-for-wavelength” rubberduckie antennas have low “radiation resistance” and squander most of your transmitter’s power on 2 meters.  Building an antenna like this one is like multiplying your power by 4-10 times! If you have a favorite repeater, or a trusted ally you need to be able to reach, and a wall that “points” roughly in the correct direction (or even within 30 degrees of the correct direction) you’re in business.
2meter actual test results: Mathematical patterns based on theory are great, but real-world results are what count. I actually tested this antenna over a 1/4 mile range (>200 wavelengths, an excellent far field test). Using uncalibrated signal strength markings on a scanner radio set to the correct frequency (this is the best field strength equipment I currently own), and the same Baofeng transmitter on the same frequency on 2 meters, I sequentially tested: (1) the supplied rubber-duckie antenna, (2) a top-rated 15” whip antenna (Nagoya NA-771 15.6” dual band) , and (3) this homemade antenna, the results were 0 (zero) signal strength units for both rubber-duckie & Nagoya whip antenna, and 8 signal units with the antenna pointed frontwards, and 6 signal units turned 180 degrees (“backwards”). This suggests moderate front-to-back ratio and wonderful forward gain over a whip style antenna–a successful 2 meter high gain antenna..
70cm actual test results: Unfortunately my current results on 70 cm are not impressive. While the antenna also works on 70 cm, I can’t promise any significant gain there; it had the same signal strength as the whip & rubber duckie. On the 70 cm band, the whip/rubber duckie are a more significant fraction of a wavelength, and therefore perform better. Reception of a local repeater was somewhat better with the yagi than the other antennas, but again the difference was not impressive.. (Improvement may be tedious, but if I ever get it done, I’ll publish how to adjust for better 70cm performance.)
Preparation for Construction
Construction should take only about 1 hour once you find the following four required items, understand the measurements, and have your tools at the ready:
- 18 feet of 12-AWG solid copper house wire, insulated (THHN) $7 (You can buy this by the foot for about $.36/foot, or you can buy a very short package of 12/2 house wiring, and get the insulated white and black wires from that. Color does not matter, but it has to have the insulation to use the dimensions below). The dimensions of the wire DO make a difference, so please use exactly this wire.
- RG58A/U coax cable with PL-259 connectors, any length from 9 to 20 feet. $7 (You choose the length to suit your needs, can be purchased from Amazon or Radio Shack, I wouldn’t go over 20 feet as the losses in the cable add up.)
- If you don’t already have it, you will need an adapter (typically $6.50 on Amazon) from your radio’s antenna connector (often, an SMA) to SO-239 (which is the “female” that fits a PL-259 “male”). ( If you have a Baofeng UV-5X type radio, the appropriate adapter can be bought from Amazon for $6.50 (see Ref  ) Other brands of radio may require the male SMA on the connector, which means they would use the adaptor in Ref , also $6.50 ) I like the pigtails to avoid stress on the fragile connections inside the walkie talkie. Be aware that SOME one-piece adapters (not the ones I’m recommending) come with “metric” threads that do not properly mate with a standard PL-259.
- 2-4 layer flat shipping type corrugated cardboard, minimum height 32″, minimum width 41″. The internal corrugation “tunnels” have to be aligned “bottom to top” not sideways, in order to position the wire elements. (Check out recycling pickup day in upsale neighborhoods and you’re likely to find plenty of this cardboard.)
You’ll need a knife, a way to strip insulation, some 5-minute epoxy glue, and some tape, preferably 2-3″ clear package sealing type tape. A yardstick or tape measure, and a way to cut the wire. A drill with a 1/4″ bit would be helpful but not required. You will need some way to secure the two required driven element connections to the coax feedline– this can be a soldering iron/solder, or crimp connectors/crimping tool, or even house-wiring pigtail spiral connectors. A bit of electrical or any other tape to cover the resulting connections if soldered.
Here’s where you get to learn a bit about antennas. A “yagi” style directional antenna makes your signal (both transmitted and received) much stronger in the desired direction, by not wasting signal or effort on some of the “undesired” directions.  It basically channels much of your transmission and reception into the “desired” sector. There is a tradeoff: the higher the gain, the narrower the “favored” region.
Energy is not “created”, its effects are just moved around smartly. To do this, the yagi has a “driven” element that is connected to the transmission line and does the initial radiation of energy. Several additional “parasitic” elements have lengths and positions that allow them to capture radiated energy and (in effect) re-radiate it with such a phase adjustment that it modifies the pattern to achieve the proper directivity.
Usually there is one “reflector” and one or more “directors”. Energy is effectively moved away from the side of the “reflector” and repositioned into the direction of the “directors”. More directors allow more gain, but there is a point of diminishing returns.
Dipole based antennas like this one usually “work” on their odd harmonics as well. This antenna uses the same driven element both on its fundamental 1/2 wavelength 2-meter resonance, and on its 3rd harmonic (70 cm band) resonance. Because of some complicated “end” effects, the point of resonance isn’t exactly 3.000 times the fundamental resonance. The current pattern in the driven and parasitic elements on their 3rd harmonic theoretically leads to some odd trifurcated forward lobes. In DK7ZB’s design, he improved the odd trifurcated pattern on the 3rd harmonic by adding additional 70cm-specific parasitic elements.
Cut the insulated wires to the proper lengths as follows. Each cut should be fairly accurate, say, within about 1/16″ of the desired length.
Driven Element: Cut two 17.5 inch sections, strip 1/4″ of insulation at one end of each, and bend 90 degrees so that there is a straight section of approximately 17 – 1/8″ and a bare tab at 90 degrees of 1/4″. You will solder/crimp/connect to the bare tabs. (See the closeup of the driven element photo.)
Remaining elements: (Leave all their insulation on)
2m Reflector: 36-3/4″
2m Director #1: 33-5/8″
2m Director #2: 32-7/8″
70cm Reflector: 11-7/8″
70cm Director#1: 11-5/8″
70cm Director #2: 11-1/2″
70cm Director #3: 10-1/4″
70cm Director #4: 10-3/4″
Mark the center of each of those elements with a marker or bit of colored tape.
Cut your cardboard to approximately 41″ wide x 32″ high. Corrugation slots on the “face” you will insert wires into must be oriented vertically, from “top” to “bottom”. If you can only find “1-ply” cardboard, use white glue to glue 3 thicknesses together to make 3-ply.
Plan ahead which direction you need to be able to aim this antenna, and how you will hang it on the wall. (You cannot use conductive WIRE to hang it, but non-conductive string or thin cord is OK. I glued a yardstick on the back of my antenna, partway down to allow reinforced holes to hold a hanging cord.)
Figure which sides will therefore be “top” and “bottom”, “front” and “rear”. Draw a horizontal “antenna centerline” 19″ from the bottom, leaving 13″ to the top. This is so the elements will NOT stick out the bottom and get bent, when the cardboard rests on the floor, until you have it mounted; the 2 meter elements will stick out the top. (Again, see the accompanying photos.) Plan to insert the 2 meter Reflector about 1″ away from the “rear” edge. Mark that point, as 0 inches. Then draw tick marks on your “centerline” at the following distances:
2m Reflector: 0″
70cm Reflector: 4-1/4″
Driven Element: 10-1/16″
70cm Director #1: 11-5/8″
70 cm Director #2: 17″
2m Director #1: 18-1/8″
70cm Director #3: 29″
70cm Director#4: 37-1/4″
2m Director #2: 38-1/16″
You should be able to locate the corrugation “slots” underneath the face of your cardboard surface. Identify the slot that goes with each of your tick marks. At the tick mark for the driven element, make two small slits 1/4″ above and below the centerline, and use these to slip the upper and lower portions in, leaving the “tabs” about 1/4″ apart. (See photo.) Make a small window, about 1/4″ x 1/4″ at each of your other centerline tick marks so that you should be able to see the center mark that you placed on every other wire at the approximate center. Measuring half the length upwards for every other element (you can use your center-marked elements as guides), make a nick and slide in the element (the 2 meter elements should be inserted from the top edge and will stick out on top). Using a drop or 2 of glue into the center windows, secure the wire elements so they won’t inadvertently drop away from their correct position.
Using the included illustration, drill or cut 1/4″ holes on the rear side of the driven element, which will allow you to feed the transmission line into a “balun coil” of 4 turns of coax feedline just back from the connection to the driven element. See the photo of the finished connection to better understand the construction. Cut off the PL-259 connector at one end of your coax cable. Starting from the “rear” end of your cardboard, feed the cable in along the centerline, and then run it through the “coil” holes to form the 4 turns of coax. This forms a “common mode inductor”; this balun (stands for “balanced to unbalance” — the antenna is balanced (symmetrical) and the feedline is coaxial (unbalanced, non-symmetrical) ) and reduces interfering radiation from your feedline.
Now carefully strip off about 1/2″ of outer coax insulation, undo the braiding & twist the fine copper braid wires into one wire; cut off any aluminum foil shielding. Carefully remove 1/4″ of center conductor insulation. Wrap the braid wire around the “down” center element’s bare tab, and the center conductor around the “up” center element’s bare tab, and solder (or crimp, or even pigtail twist). If you solder, insulate the result with a bit of electrical tape. Use clear packing tape to secure the center connections about 1/4″ apart.
You’re finished; connect the distant PL-259 to the pigtail adapter to your handheld transceiver and have fun!
APPENDIX: HOW I GOT THIS DESIGN & ADJUSTED IT
(Some heavy duty theory)
I got the original dimensions and spacings for this antenna from an open-air boom-based yagi design published by DK7ZB.  I adjusted the lengths and spacings and built it by sliding wires into the corrugations of cardboard — and it didn’t work! Something was haywire. SWR’s (standing wave ratio) were terrible. Thankfully I have a wonderful measurement device (Ref ) that allows me to read resistance and reactance of the antenna on the 2m band (but won’t reach the 70 cm band). Temporarily using a very short 2 foot transmission line allows the instrument to closely read the actual antenna (longer transmission lines can change the apparent resistance/reactance of the antenna, and create baffling results that remind you of high school questions about resonances of open and closed-end flute tubes). Starting over with just the driven element showed the resonance was far below the calculated 2-meter band resonance– actually down at 131 MHz. Additional experiments confirmed that the dielectric effects of both the THHN insulation and the cardboard were making the wire “appear” electrically to be 7% longer. The resonances observed in a few experiments were as follows:
Resonance Effects Measurements
38.0″ dipole bare wire, free space: approx 142 MHz resonance (R = 76, X = 0) [close to expected]
38.4″ dipole insulated wire, free space: approx 135.5 MHz resonance
(Although the wire was 1% longer, the resonance moved a full 5% lower; 4% lower resonance apparently due to insulation)
38.4″ dipole insulated wire, inside cardboard: approx 131.4 MHz resonance
(Additional 3% drop in resonance due to placement of insulated wire in cardboard.)
After this experiment, I adjusted all the wire lengths (7% shorter than expected for 146 MHz or 440 MHz as appropriate), rebuilt the antenna (to the dimensions given here) and the 2-meter antenna worked perfectly with an acceptable SWR around 2:1, a good bandwidth, and great forward gain (measured experimentally as shown below). Because of the influence of the parasitic elements, the resonant impedance of a yagi may not be 50 ohms, hence perfect SWRs do not always result. The 70 cm antenna had a wonderful SWR but the gain is not impressive. This can likely be improved experimentally by adjusting the position of the director elements, one by one, with wooden tongs while measuring the far-field strength. This will be tedious and I don’t have the time to work on it right now.
If you want to measure SWR on these bands, consider purchasing the Workman 104 VHF/UHF swr bridge ($44) , which also give an approximation of power measurement. 
 For an example of just how much worse a short stubby “rubber duckie” antenna can be, compared to a properly sized antenna, see: http://blog.g4ilo.com/2011/04/2m-ht-antenna-shootout.html In a quasi-professionally done measurement, he observed enormous losses in very short antennas at 2 meters.
 For Baofeng-style walkie talkies: SMA Female to SO-239 Female 6″: Available from amazon.com at: http://www.amazon.com/gp/product/B00COKNKS8/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=1944687762&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00ANDSL96&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=178TD40ZXERF5ZF5P4H8
 Adapter for radios with SMA-female connector: http://www.amazon.com/Handheld-Antenna-SO-239-Female-Connectors/dp/B00COW5E3A/ref=sr_1_1?s=pc&ie=UTF8&qid=1440594046&sr=1-1&keywords=SMA+male+to+SO-239
 http://www.qsl.net/dk7zb/Duoband/4+5_2m-70cm.htm Author showed that even his 2meter yagi functioned reasonably well on 70cm; adding specific 70cm directors improved the forward pattern.
 VHF/UHF swr bridge: http://www.amazon.com/SWR-Power-METER-VHF-Radio/dp/B002KYE190
Prizes for this round (ends October 11 2015 ) in our non fiction writing contest include…
- First place winner will receive – Two Just In Case… Essential Assortment Buckets courtesy of LPC Survival a $147 value, a Wonder Junior Deluxe Hand Grain Mill courtesy of FoodPrepper.com a $219 value, and a gift certificate for $150 off of Rifle Ammunition courtesy of LuckyGunner Ammo… Total first place prize value over $516 dollars.
- Second Place Winner will receive – A case of Sopakco Sure-Pak MRE – 12 Meals and a Lifestraw Family Unit courtesy of Camping Survival.com, and a One Month Food Pack courtesy of Augason Farms.com
- Third place winner will receive – $50 cash.