Build your own 2m/70cm 3 Elements VHF by 5 Elements UHF Yagi antenna with excellent gain for both VHF and UHF operation. This antenna is designed for amateur band frequency for both VHF (2m) and UHF (70cm). The antenna has good reflection coefficient and VSWR ratio @ 1.2:1 at center frequency and 1.7:1 at band edge.
Antenna Specs from 4Nec2
VHF Gain: 7.25dBi @ 145Mhz
Beamwidth: 114° degrees
Front to Back Ratio: 14.1dB
Expected Pattern VHF
UHF Gain: 10dBi @ 440Mhz
Beamwidth: 60° degrees
Front to Back Ratio: 14.4dB
Expected Pattern UHF
Building Video 3 Elements by 5 Elements 2m/70cm Dual band Yagi including Tuning
Material list for 3×5 Elements Dualband Yagi
1. 1″ X 0.5″ Rectangular Aluminum tubing for the boom
2. 3/8″ Aluminum tubing for antenna elements
3. 1cm Outside diameter antenna tubing for elements holder
4. 2pc SO239 connector
5. Pop rivets / Rivet tool
6. #12 AWG Copper wire with insulation (12″for Gamma match)
7. Soldering iron
8. 10pcs Stainless steel nuts and bolts 20mm length 3mm diameter
9. 1pc, Butterfly nut and 1 bolt 18mm length 3mm diameter
10. Aluminum plate 0.5mm thickness
11. 4 X 2″ Aluminum tube for bracket
12. Collapsible tube (shrinkable tubes)
QYT KT8900 has good form factor that fits in the palm of your hand. It’s a dual band radio 2m/70cm that supports dual frequency monitoring has an acceptable receiver capability for the price, good audio performance (loud and crispy). Supports FM radio broadcast listening moreover it has a built in repeater function that will work in combination of another QYT KT8900 or pair it with QYT KT8900D by just enabling the function on the menu system and adding a repeater interface cable which can be built easily with spare LAN cable, RJ45 connectors and crimping tool.
If it’s not for the faulty heat sinking then probably this radio will work longer than expected rather than giving up the magic smoke of the RF finals too soon.
Preparing the RF finals removal and replacement
Disassembling the radio to replace the broken RF finals reveals that the screw holding the heat sink is loose just secured by a piece of copper strip inserted to the metal spacer to float the heat sink above the RF finals.
Replacing the RF finals is moderately easy provided you have the right tools a hot air rework station, solder flux, a pair of tweezers. The radio board backside reveals a slightly bigger heat sink which probably is intended to absorbed the heat more effectively by coupling it on the radio chassis, but without using a heat sink compound this also is not too effective to properly address the heat issue.
Time to replace the RF finals, apply the solder flux on the chip, heat the board just enough for the solder to melt at the bottom of the RF finals, remember that this is also attached to a copper heat sink below so heating it will take a little longer before you can see that a solder is flowing on the side of the finals. Once removed successfully replaced the finals with a new one. Good news for people living in the Philippines, the RF finals is now available by visiting your favorite electronics store in Gonzalo Puyat street in Sta. Cruz Manila price varies depending on the stores.
Preparing the fan modification on the radio chassis
Attaching the fan on the radio chassis requires a bit of drilling and tapping to hold the fan securely at the back of the chassis. For QYT KT8900 you can see at the back of the chassis that they somewhat prepared a fan mounting holes, but they didn’t pushed through with the plan. On later versions of this radio a fan is now attached to help in cooling the radio.
Drilling the fan mounting hole with a drill step bit.
Preparing the screws holder by tapping a thread.
Putting it back together.
Finally the finished modification applied to the radio.
The final look and successful cooling fan modification.
QTY KT8900 and QYT KT8900D chinese radios and variants have a built in repeater function that works out of the box without additional modification on the radio. You just need to enable the repeater function (cross band repeater VHF/UHF or UHF/VHF) via the menu system. Connect the two radios via the mic port using the cable shown here and enable the repeater function on the radio. The radios will work in the repeater mode cross band and will re-transmit your audio either on the VHF or UHF and vice versa.
The repeater interface will work on the variants of these radios in either combination of at least 2 KT8900 RX/TX,
2 KT8900D RX/TX or 1 KT8900 and 1 KT900D and of course Baofeng Tech radios. The configuration is done on individual radios by setting the REP-M (Repeater transponder function on both radio). Once a matched Carrier, CTCSS/DCS, TONE or DTMF is received in either of the radios it will re-transmit the audio on the other radio and vice versa.
Build the cable
Do it yourself using RJ45 modular connector and a piece of LAN UTP cable
Finished Cross Band X-Band repeater interface
Let’s see how it works
Testing video of working repeater system
1. Easy deployment for field work to extend portable radio range with acceptable results
2. Will fit easily in a go box
3. Inexpensive but it works
1. Recommended for light usage as QYT Radios tend to heat up easily
2. Use the upgraded version of QYT the KT8900D for more stability
3. Suitable only for Cross Band configuration
3. KT 8900 heats up like a barbecue grill if it didn’t burn your finals at least bring a hotdog to grill …
3 Elements Yagi optimized for 2m 146Mhz. The design is very similar to 3 Elements Yagi for 145Mhz for the boom and elements spacing. The only difference is on the cutting of the elements. Check the design for 145Mhz Yagi here.
Like most of my design this is an end mount type yagi which is more efficient than a mid mount type yagi as the mounting will not introduce a pattern distortion.
Similar materials are used in the construction:
3 Elements Yagi Antenna Materials List
1″ X 0.5″ Rectangular Aluminum tubing for the boom
3/8″ Aluminum tubing for antenna elements
1cm Outside diameter antenna tubing for elements holder
1pc SO239 connector
Pop rivets / Rivet tool
#12 AWG Copper wire with insulation (12″for Gamma match)
6pcs Stainless steel nuts and bolts 20mm length 3mm diameter
1, Butterfly nut and 1 bolt 18mm length 3mm diameter
Aluminum plate 0.5mm thickness
Collapsible tube (shrinkable tubes)
To check the antenna pattern and expected gain if the antenna will perform similarly on other bands these are the results with both slight increased in SWR and little decreased in gain on 144Mhz and 148Mhz respectively but the expected pattern are generally the same. I’m using 4NEC2 for antenna simulation and analysis.
Predicted Antenna Pattern for 146Mhz
Predicted Antenna Pattern for 144Mhz
Predicted Antenna Pattern for 148Mhz
For SWR curve this antenna exhibits a pretty wide band performance on 4NEC2 simulation.
Sleeve Dipole Antenna
Build a 2m VHF lightweight sleeve dipole antenna. A compact portable antenna and easy for deployment. You can even operate the dipole while holding it. Building it is easy just follow the antenna plans below. The antenna specifications are below including the testing video.
Antenna Gain: 3dBi
Features: Lightweight and Portable easy deployment
Building a 15 Element Yagi for 2.4Ghz Wifi frequency. This antenna build is very useful for extending your wifi range indoor or outdoor. The approximate gain of this 15 element yagi is approximately 13dBi with a good front to back ratio of 20.6dB. Beamwith is 40° horizontal and 70° vertical.
1. 12mm x 8mm uPvc moulding (Boom)
2. #12AWg solid wire for elements
3. Measuring tool / precision cutter
4. 1 SMA female connector
5. Coaxial cable suitable for SMA connector
6. Sandpaper or file tool for removing rough edges of the elements
Gain and and antenna patterns are from 4NEC2 antenna modelling software.
Some direct measurements from neighbors WiFi access point:
This first screen shot reading is from a WiFi card with a 5dBi dipole antenna
The second screen shot is taken from 15 Element Yagi antenna build taken indoors
As we can see from the above screen shots the initial measurement from a dipole yields -88dBm our reading from the yagi -76dBm direct calculation from the 2 values ( -88dBm – -76dBm = -12dBm) our yagi gain is almost equal to the theoretical gain of 13dBi. The above measurements are taken indoors the WiFi card is connected to a laptop.
So you follow the tutorial article on how to make the 3 Element Yagi on this page. You’ve even watched the tutorial video on how I build the 3 element yagi and wanted to duplicate it. You’re all set with all the materials at hand and most importantly you’ve got the perfect time to do it now. So you proceeded with the project, you cut the elements, marked the spacing, mounted the elements holder and things are looking good. Except for the last part the diagram seems too complicated to follow. So worry not this is the tutorial video on how I build the gamma match for my antenna build.
So to recap:
The gamma accomplishes 3 things:
1. Usually it’s a small diameter wire parallel and in close vicinity with the main radiating element, it will carry only a fraction of the main element current while being exposed to the same electrical field strength. This turns it in an effective up-transformer of the antenna input impedance. A sort of folded dipole performing an impedance step up.
2. It forms together with the main radiating element a closed wire stub, adding inductance to the antenna input impedance. If it is not required for matching, this additional inductance can be cancelled out with a lumped capacitor in series. A parallel shorted transmission line stub, adding shunt inductance.
3. The sheath of the coaxial feed-line (braid) is connected to the center of the main radiating element. When properly connected, a gamma-match also serves as a balanced to unbalanced converter or balun.
Here’s the how to video:
If you want a more in depth discussion and mathematics behind the gamma match read and download the gamma match document below.
Let us try to answer the question above. How to program VHF channel on Baofeng BF888s UHF radio and will it be useful?. So on the first question, can we program VHF channel on BF888s UHF radio?. Fortunately yes it seems that the radio chip on Baofeng BF888s are capable of accepting VHF frequency channels although I have not open the radio to check the datasheet on what chip was actually used on this tiny radio with a good form factor.
Now that we are sure that the radio can accept both VHF and UHF frequency, let’s move to the second question, will it be useful?. Let’s answer that by watching the how to video and let’s give out the conclusion later on.
So let us now give the conclusion after watching the video playlist. Will it be useful after programming VHF channel on it?. My answer is a YES and NO, yes it will be useful for monitoring radio frequency on VHF channel and no it will not be useful for field work as the measured output power on VHF is less than 1 watt for it to be useful on normal field work. Although I must admit the radio has a good form factor and suitable for short distances communication office or home work.
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