Dr. FB

This is the third and final part of the article for antenna performance of handheld transceivers. In the first part of the series, we explained how to measure the SWR of an antenna attached to a handheld transceiver. In Part 2, we actually measured the SWR of three antennas purchased in Taiwan, and one flexible antenna that came with the handheld transceiver that we are using as our jig. In the last part of Part 3 of this article, we verify which one of the antennas performs the best for transmitting, and verify whether it can also receive signals well, using the results of our field tests.

Field test scene

1. Field test preparations

Field tests were performed using the configuration shown in Figures 1 and 2, shown below. Attach one of the four antennas to be measured to the ID-51, and transmit with 0.1 W (SLO) power. These four types of antennas are the antennas introduced in Part 2.

dBm is the unit of measure of the signal strength at the receiving point, measured by a spectrum analyzer. dBm is an absolute number. The scale of the graph showing the result is also displayed in dBm. However, the absolute value is not important in these tests. The measured value is simply regarded as a comparison value, because it is a comparison of four antennas. At the end of this article, we summarized the measurement data and results comparing the SWR characteristics versus the signal strength of the 144 MHz and 430 MHz antennas, and the radiated power of those antennas.

2. Transmit Performance test (Field test 1)

Field test 1 compares the signal that is radiated from each antenna. To measure the signal radiation, check the electric field strength at the receiving point with a spectrum analyzer. One of the four types of antennas were alternately used to test on the transmit side. The antenna that radiated the strongest signal was judged to be the best antenna. The antenna on the receive side was not changed.

Figure 1. Transmit antenna performance test (Field test 1)

3. Receive Performance test (Field test 2)

Field test 2 confirms whether there is a difference in reception, even with an antenna purchased in Taiwan. As shown in Figure 2, the transmitted signal is measured using four types of antennas to measure the received signal strength. At this time, the transmit antenna was not changed, but the receive antenna was replaced.

Figure 2. Antenna test as a reception antenna (Field test 2)

4. Verification on Field tests 1 and 2

The measured antenna is a dual band antenna for VHF/UHF handheld transceivers. The results of field tests 1 and 2 were compared vertically as VHF and UHF data, respectively. The left column is VHF and the right column is UHF. The SWR characteristics, transmit antenna characteristics, and receive antenna characteristics are listed in order from the top, and if you look vertically at the data, you can see the antenna performance for transmission, and the performance as a receive antenna comparing the SWR and frequency.

Left: Figure 3(a) SWR on VHF, Right: Figure 4(a) SWR on UHF

Left: Figure 3(b) TX antenna test on VHF, Right: Figure 4(b) TX antenna test on UHF

Left: Figure 3(c) RX antenna test on VHF, Right: Figure 4(c) RX antenna test on UHF

(1) VHF antenna measurement result

The antenna measured was for a VHF/UHF handheld transceiver, so the measurement was also performed using an ID-51 VHF/UHF D-STAR transceiver.

Compare Figure 3 (a) to Figure 3 (c) in the left column. From the graph in Figure 3 (a), the SWR value on VHF was not as low as expected for any of the four antenna types measured.

There are 5:1 to 6:1 SWRs on the VHF band, except the flexible antenna. When the flexible antenna is attached to the ID-51, and then you transmit, the electric field strength at the receiving point is as shown in Figure 3 (b). You can see that the difference in the electric field strength, depending on the frequency, is very large. In addition, you can see that the signal strength of any antenna other than the flexible antenna decreases at around 145 MHz. Dr. FB also learned this for the first time in this survey.

I think there is a frequency at which the resonance point of the ID-51 chassis and the antenna is at its worst, but no further verification has been made with this test. It is an interesting result to be concerned about. For reference, when transmitting, if you hold the transceiver on the transmitting side in your hand, the dip point (reduced point) of the signal strength on the receiving side changes about 500 kHz. You can see that the human body is affecting the ground.

Figure 3 (c) shows the result when the antenna under test is used on the receiving side. The difference in received signal strength also seems to be correlated with the SWR graph in Figure 3 (a). In short, it turns out that a good SWR antenna gives good results in reception. Since the ground of the 1/4λ antenna is the chassis of the transceiver, it seems that the ground is not sufficiently obtained in a small area chassis.

(2) UHF antenna measurement results

The VHF/UHF dual band antenna prepared this time has a 1/2λ configuration in the UHF band, so the grounding of the transceiver does not significantly affect the antenna SWR. The three graphs in Figures 4 (a) to 4 (c) clearly shows this. Although there are individual differences, depending on the antenna, we cannot see a big performance difference from the current tests that the SWR differs greatly, depending on the frequency, or that it has a significant effect on the transmitted radiation or received signal strength. We imagine that the UHF band is not affected by the ground because antenna used is a 1/2λ type.

Let's take a closer look at the test results. The NA-771 antenna shown in the red graph shows an SWR near 1:1 in the band, according to Figure 4 (a). It can be seen in Figures 4 (b) and 4 (c) that this antenna has a good effect on both transmit and receive. From these measurements, it can be said that the UHF band has almost uniform characteristics within the band.

In transmit, the signal strength difference between the flexible antenna and other three antennas is about 5 dB, according to Figure 4 (b). The signal strength at the receiving point when transmitting at 433.00 MHz using a flexible antenna was -40 dBm. When the NA-771 antenna was connected instead of the flexible antenna, and then a signal transmitted, the signal strength at the receiving point was -35 dBm, and the signal strength increased by 5 dB by replacing the antenna. I have not confirmed the communication distance in this field test for the difference of 5 dB.

In the Figure 4 (c), it can be seen that the difference in signal strength when using the NA-771 antenna for receiving is about 5 dB compared to when receiving with other antennas. In fact, when a weak D-STAR signal was received with a flexible antenna, the signal’s audio became R2D2 sounding, and the signal could not be properly received. When the antenna was replaced with this NA-771 antenna, the R2D2 voice sound disappeared and clear reception was possible.

5. Summary of field tests

Three VHF/UHF antennas purchased in Taiwan have been verified in this article, but in short, when used as a VHF antenna, performance was not very good, unfortunately. Rather, you can see that the attached flexible antenna performs well. In addition, it has been confirmed that the reception sensitivity improves when the handheld device is held in the hand at the time of VHF measurements, and the signal strength reaching to a station increases.

On the other hand, in the UHF band, the SWR value, the measurement result of the reception and the transmission all show good results, and this also agrees with the theory that the 1/2λ antennas are not affected by the earth (ground).

6. Thoughts of Dr. FBs

The coronavirus infectious disease (COVID-19) is rampant worldwide and is in a serious situation. We are sincerely sorry for those who have passed away. We would like to offer sympathy to all those affected.

Thank you for reading this article. I guess you are unable to go to work and are restricted from going out to prevent the spread of this coronavirus infection. Dr. FB, wishes that this plague could end as soon as possible around the world, reduce the turmoil in the world, and enable you to act freely. God bless you.

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