T3 internal UHF radio return loss

John Thompson

Well-Known Member
In setting up my T3 to transmit corrections via UHF with the internal radio, I noticed that the antenna impedance match is pretty poor. (2-4 dB return loss) I tried 3 different antennas with right angle SMA connectors, all with similar results. I think much of the problem is the T3 case being right next to the antenna. If the antenna is pointing down instead of up, the match is much better. Pointing the antenna down might give better range and will probably make the transmitter happier. I think the best return loss I saw was 12 dB with the external antenna with a long cable and TNC to SMA adapter. Are other people seeing better impedance match than this?
 

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I am interested in the proper position for the UHF antenna. Several other brands with fixed UHF antennas have them pointing down which I always thought a bit odd but perhaps that is the correct way for best range after all.
 

John Thompson

Well-Known Member
I did a bunch of testing of UHF radios the last couple days. I made a test matrix to try a bunch of combinations.
I tested the 1 Watt T3 internal UHF radio and HPT404BT transmitting at 1 Watt and at 4 Watts.
Transmit antenna combinations included right-angle SMA antenna in various orientations and 2.4dBd antenna with 12' cable (with and without ground plane).
Base-Rover distances ranging from 100ft to 17,290ft (3.3 miles).
Internal and external receive antenna on T-LS+

All tests were at 464.55 MHz, D8PSK, 12.5 kHz bandwidth.
Unobstructed lines of sight between transmitter and receiver with open skies.

RSSI values at 100ft were as expected when using the T3 with 2.4dBd antenna on the transmitter, but were 11-17dB lower than expected when using the right-angle SMA antenna, depending on orientation.

I tested the right-angle SMA antenna pointing up and pointing down. I also turned the T3 so that the antenna was on the side toward or away from the rover. In all cases the T-LS+ rover was turned so that the external UHF antenna is on the side toward the T3 base. At 100ft, pointing the antenna down and on the side toward the rover gave a 4-6dB gain over the other orientations, but antenna orientation made little or no difference at longer ranges.

The 2.4dBd antenna gave a consistent 9dB gain over the right-angle SMA antenna.

The return loss of the right-angle SMA antenna jumps around between 2-4dB when it's pointed up and between 4-22dB (averaging about 7dB) when it's pointed down. I used a 10" fine-tooth saw blade for a ground plane for the 2.4dBd antenna. It had no significant effect on RSSI, but Return Loss is a little better with it, 8dB instead of 7dB, both steady.

I'm pretty sure that performance would be improved if the right-angle SMA UHF antennas on the T-3 and T-LS were upgraded, maybe to these.

My T-LS+ apparently does not have an internal UHF antenna switch or internal UHF antenna. Switching the software between Internal and External makes no difference to RSSI. If the external antenna is connected, the T-LS+ works just as well with Internal selected as with External. If the external antenna is not connected, the T-LS+ works just as well with External selected as with Internal. The external right-angle SMA antenna on the T-LS+ adds about 15dB to the RSSI.

Here is what I got using the T3 internal transmitter (transmitting +30 dBm, 1 Watt) with the 2.4dBd antenna (antenna was on a 12-foot pole) and the external right-angle SMA antenna on the T-LS+:
Feet dBm RSSI
100 -31
800 -51
2,000 -78
4,000 -82
6,776 -86
10,132 -87
17,290 -88 This was hilltop to hilltop, about the maximum range with direct line of sight to my Tx location.

At that maximum range, I tested the minimum power needed to get usable RTK with and without the external antenna on the T-LS+.

Without the external antenna on the T-LS+, I was able to get usable RTK corrections at 17,290 feet (3.3 miles) transmitting 1W with the 2.4dBd transmit antenna. RSSI was -105dBm.

With the external antenna on the T-LS+ I was able to get usable RTK corrections at 17,290 feet (3.3 miles) transmitting +15dBm (30mW), though it was struggling some at that level. RSSI was -107dBm.

Minimum RSSI needed for RTK is around -100dBm.

These numbers represent near ideal conditions: clear lines of sight, no hills, trees, or buildings in the way. The practical range limitation is terrain and other obstructions. Put your transmitter antenna on high ground with a tall tower.
 
Thank you for that very in-depth experiment. I feel like $55 is well spent to get a little more range. I really like the "lean and light" approach to my field work and don't want to change my 90 degree external antenna on the LS+ to one sticking out the side but it would be easy to do for my base.
 

Jim White

Member
I wonder if the antenna up vs down may be a function of the distance between the antenna and the ground vs. the wavelength. You might try different heights to see how it affects the range. I believe the wavelength of 464.55mhz is 2.1feet.
 
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