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While searching the web I came across some cheap yagis HDTV antenna, which claim to operate from 45 to 860MHZ, have a 150 miles range ( 22-38dB gain) and come with a rotator. All this for for $32.99 including shipping. I wanted to see how well would these antennas work on the 2m/70cm band, and see if I could modify them to work better on these bands. The particular model I got is the ESky HG-981 from amazon. Here are the specs for this antennas: - Frequency: 45-860 MHz Before assembling it, I took it apart and measured everything on it so I can replicate it in an antenna simulator to see what I need to change. There is only a single coaxial cable leading into the antenna, which contains a filter to pass 12V AC into the motor, and filter out all other higher frequencies into the antenna. Before going into the radiating elements, the signal goes though another filter (a diplexer) which seems to split VHF and UHF signals into separate radiating elements from the signal feed line. The VHF radiating element is a folded dipole, while the UHF looks like a dipole with two fingers for radials. I then reproduces the antenna in Auto-desk Inventor to capture all of its parameters. Here is a PDF file with all the dimensions. The antenna seem to be composed of a reflector dish for 2M and a yagi for 70cm. {youtube}gxRcYO6uhJM{/youtube} Here is the antenna mounted on a test pole I have on the balcony. I took some preliminary measurements using by VNA as well as having some people from the CVARC ham club send some test transmission (Thanks for TIM-K6POI Todd-KD6RCM and Zak-N6PK for helping out). Since the impedance if 75 Ohms I tried a quick transmission but did not measure anything (only Tim was able to hear me on 2M). Here are the results from the VNA on 2M and 70cm. As can be seen the VSWR is very bad on these bands (as expected from the impedance mismatch). 70cm looks much better, but still not very good.
Ham / AntennasIntroduction
- Gain: 22-38dB
- Rotation: 360°
- Channel: VHF1-12 UHF21-69
- Impedance: 75Ω
- Power: 3W
- Power Supply: AC110V
- Rotation speed: 2-4 rounds/min
Testing
Here are the results. Tim's station was 7.12889km away from the antenna at about 130 degrees to the antenna.
Angle
Test1@60W (dBm)
Normalized (dBm)
Test2@60W (dBm)
Normalized (dBm)
Test3@20W (dBm)
Normalized (dBm)
0
-104
-5
-105
-7
-112
-6
25.7143
-104
-5
-105
-7
-113
-7
51.4286
-103
-4
-104
-6
-112
-6
77.1429
-102
-3
-103
-5
-109
-3
102.8571
-100
-1
-101
-3
-107
-1
128.5714
-99
0
-100
-2
-106
0
154.2857
-100
-1
-98
0
-106
0
180
-103
-4
-99
-1
-109
-3
205.7143
-113
-14
-103
-5
-114
-8
231.4286
-115
-16
-108
-10
-116
-10
257.1429
-111
-12
-116
-18
-117
-11
282.8571
-109
-10
-114
-16
-115
-9
308.5714
-108
-9
-108
-10
-116
-10
334.2857
-107
-8
-107
-9
-115
-9
Here is a normalized polar plot for the 60W transmission. 
Since I did not measure the power on a reference dipole antenna, I could not determine the gain correctly.
Theoretically, using Friis equation (taking the log to simplify things), and a theoretical 2.15 dB half dipole, I should be getting -38dBm.
TX Power (dBm) + TX Gain (dBi) + (-32.44 - 20 log(Freq MHz) - 20log(Dist km) ) + RX gain (dBi) = RX power (dBm)
47.781513 dBm + 4.5 dBi + (-32.44 - 20* LOG(146.55,10) - 20*LOG(7.12889)) + 2.15 = -38dBm
Which is very far from the -98dBm I was getting. This is probably because of all the interference between us, so next time I would need to measure the actual received power on a dipole.
Simulation
The next step is to simulate this antenna using NEC2 so I could determine what I would need to adjust inorder to improve the antenna for use on the 2M/70cm band.
Here are some preliminary simulations results, which I am still working out to see if they are correct.
Here is the NEC file I am using.
2M simulations results
70 cm simulation results
