Since purchasing my Retevis RT85 2m/70cm handheld radio I’ve noticed that it seems rather deaf when using the antenna that came with the radio and isn’t as strong into the local repeaters as I imagined it would be.
Considering the local 2m and 70cm repeater isn’t that far from my QTH and there is pretty much a clear line of site view in the direction of the repeater I was somewhat surprised that on 70cm the repeater never breaks the squelch, even if it is set on it’s lowest setting of zero.
Connecting my home made end fed dual band vertical dipole at 10m above ground the performance of the radio improves drastically as one would expect.
Having recently purchased a JNCRadio VNA 3G antenna analyser I decided to connect the Retevis supplied antenna to the analyser and see what the resonance was like on the two bands.
The antenna is labelled as 136-174Mhz and 400-470Mhz. This is an extremely wide frequency range for such a small antenna and clearly isn’t going to perform that well over such a wide bandwidth.
Connecting the antenna to the VNA and setting the stimulus frequency range to 144-148Mhz I found that the SWR curve of the antenna wasn’t particularly good.
As shown above the SWR curve on the 2m Band is pretty poor. At 144.0Mhz it’s just over 3:1, at 145.496 (closest I could get to the 145.500 calling channel) the SWR is still 2.1:1. The antenna doesn’t really get close to resonance until 148Mhz where the SWR is 1.46:1.
With an SWR this high the radio will almost certainly be reducing the O/P power considerably to protect the PA stage from over heating due to so much power be reflected back into the transmitter. This explains the poor performance when using 2m repeaters locally and the somewhat limited range when using the OEM supplied antenna.
Looking at the SWR curve on the 70cm band, the antenna is much closer to resonance than it is on the 2m band but, it’s still not perfect.
At 430Mhz the SWR is 1.56:1, at 435Mhz 1.63:1 and 440Mhz 1.72:1. Since the antenna is much closer to resonance on the 70cm band I would expect it to perform better than it does.
Looking at the SWR curves over the entire supported frequency range of 136-174Mhz and 400-470Mhz, there is only one point of resonance on VHF around 148Mhz and on UHF around 400Mhz.
With such disappointing performance on both VHF and UHF I’ve decided to investigate making my own 2m/70cm antenna for the handheld to see if I can improve both the SWR on each band and the overall performance of the radio.
More soon …