Adaptation of 75 ohm RF amplifiers to 50 ohm VNA for measurement - Page 1
Forum section: electronics / rf-microwave

author: MF-jockey
author_rank: Regular Contributor
timestamp: November 24, 2020, 06:18:09 pm
content: I needed a 75 to 50 ohm adjustment and more than 20 dB attenuation for measurements on RF-amplifiers (LNA for house antennas) with 75 ohm connections.So I take a "DC-4.0GHz RF Fixed Attenuator" (you can buy it from Ali for about 4 €,https://www.aliexpress.com/item/4000333427604.html?spm=a2g0o.detail.1000023.1.26f13010qDiC5r) and supplement it with resistors and F-sockets.I also added capacitors to protect the VNA against DC voltages. They would be better at the F-sockets, but there wasn't enough space.The reference loop measurement with nanoVNA resulted in an attenuation of 27.1 dB, ideal for my purposes.The modified 20 dB attenuator was connected between port 1 (TX) of the nanoVNA (V2_2) and the Input of the RF amplifier. This avoids overdriving the amplifier.The modified 0 dB attenuator (now 5,6 dB, seehttp://ham-radio.com/k6sti/match.htm) was connected between port 2 (RX) of the nanoVNA and the Output of the RF amplifier.One of the measurement results, the nanoVNA sees exactly 50 ohms.

author: radiolistener
author_rank: Super Contributor
timestamp: November 25, 2020, 03:07:57 pm
content: there is no need capacitor. NanoVNA don't have DC component on the output. Such capacitor just make your measurements more complicated, nothing more.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 25, 2020, 03:51:20 pm
content: If I understood right, he wants to use such a LNA with F sockets normally fed over the coax by e.g. sat receiver, so it makes sense to block the DC; there are also F DC-blocking adapters for this purpose on the market. In general I wouldn't bother too much about impedance matching; those LNAs normally work quite nicely in a 50 ohms environment

author: radiolistener
author_rank: Super Contributor
timestamp: November 25, 2020, 04:01:50 pm
content: Quote from: HB9EVI on November 25, 2020, 03:51:20 pmif he planning to use LNA for receiver, then attenuator is not needed at all, it will reduce sensitivity and dynamic range. Nothing more.If signal is strong enough to use attenuator, then LNA is not needed.

author: MF-jockey
author_rank: Regular Contributor
timestamp: November 25, 2020, 09:46:56 pm
content: So that no misunderstandings arise, I would like to give a few more explanations.The first measurement object was a multi-range amplifier for antenna distribution in a larger house.The amplifier has 3 branches:1. FM radio (87.5-108 MHz)2. DAB + radio (174-240 MHz)3. DVB-T2 TV (470-790 MHz).There are also options for direct voltage supply to preamplifiers (DC risk!).A total of 12 antenna sockets are attached to it and also the terrestrial feed into a 5/8 Sat Switch.An F DC-blocking adapter is coming soon. If you accidentally forget it, the soldered capacitor protects the VNA.The risk occurred to me a little late, otherwise the capacitors would be right behind the F-sockets.With the current solution, I only risk the (easy to change) resistors of the attenuator, but not the VNA.The lower limit frequency must of course be observed.And yes, I measured the antenna junction boxes directly, with SMA to F adapters. That works fine.In order to adjust the L C input filters of antenna amplifiers, I would like to have the best possible impedance matching.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 25, 2020, 10:46:51 pm
content: all fine; just place the LNA as close as possible to the antennas to amplify the signals and not the noise of the cable