China 50 Ohm Loads

The Celwave dual stage isolator used for the new repeater was obtained at a local Hamfest sans it’s termination loads. So it really was a circulator at that point, and I needed some 50 ohm loads for this isolator application. My past experience with Chinese terminators of the low power variety has not been good. The ones I tested before, rated for 1W and 2 GHz, were pretty terrible at higher than VHF. Even at 145 MHz they only had about 15-20 dB RL depending on the sample. At anything over 500 MHz they were totally unusable unless you were trying to simulate a very poorly matched antenna.

With that in mind, I reluctantly ordered a 10W and 50W load from China for the isolator. I wasn’t expecting much. The price was cheap enough and I figured on replacing them with something better when decent ones could be found at a future swapmeet. The 10W load was about 13 dollars shipped and the 50 watt about 27. They arrived pretty quickly as China goes, and their external finish and quality seemed high. The question was how good of loads they were.

Surprising good..

10W China Load
50W China Load

Way better than I was expecting. 38 and 42 dB RL at 450 where they were going to live. The isolator was very happy. With the good antenna match at the site, they shouldn’t see a lot of action from the repeater TX reflected power, but will hopefully shunt the other guy’s RF to ground and keep it out of the transmitter.

50W China Load from eBay

Dave, K7DMK

Inexpensive High Performance Preamp

The Qorvo SPF5189Z based RF gain block modules being sold on Ebay for about 5 dollars shipped, offer an excellent value for building a low cost VHF or UHF preamp without sacrificing performance.

These are sold as a small double sided PCB with female SMA connectors installed. The bottom of the board is a continuous ground plane and the top side has a small tin plated shield over the RF circuitry. They require only a single +5 volt supply at about 100 milliamps for operation. The active device is the SPF5189Z which is a GaAs pHEMT low noise MMIC amplifier.

$5 Preamp module

The specs for this device are very impressive for it’s price point. We can thank the cell phone industry for high performance, high volume (cheap) parts like this. The MMIC is rated for gain from 50-4000 MHz. At 70cm the gain is about 20+ dB and at 450 MHz. the noise figure less than 0.5 dB. A quick summary of the SPF5189Z is below:

Qorvo SPZ5189Z

These specs are very similar to to the highly rated and more expensive PGA103+ amplifier MMIC from Minicircuits. It has been the part used in many high quality Ham preamps being sold today.

The down side of these modules is they are broad-band, untuned circuits, and require filtering and/or additional shielding in high RF environments.

The preamp employed on the new K7EVR repeater is build with one of these modules. It was mounted in a small Pomona size B diecast shielded box with robust M/F type N connectors installed. The SMA connectors were removed and short heavy gauge wire ran from the RF in/out on the PCB to the N center pins. These leads ended up being about 10 mm long, and significantly degraded the input return loss of the preamp over 1 GHz.

Homebrew Preamp

As can be seen, RL is about 20 dB at 450, but gets worse from there. The bottom side of the board is grounded to the case and additional ground paths to the SMA shield points are provided. This is OK for 450 MHz; not OK much higher in frequency, with the added inductance. It was just luck that in this case, the minimum SWR was near the desired operating frequency. Clearly, a better remounting arrangement is needed for microwave frequencies. Still, compared to the average GaAs FET preamp with it’s typically poor in-looking RL, the input VSWR of this MMIC is great. No complaints..

Input filtering to the preamp used on the repeater is furnished by two series connected six inch diameter cavities that provide a 3 dB bandwidth of about 400 KHz. This also adds about 40 dB more TX isolation to the 110 dB notch provided by the duplexer. At these levels, shielding is everything in order to actually achieve isolation close to this magnitude. In this case, the preamp’s N connector screws directly to the output of the cavity filter, and the two cavities and RX interconnect are Superflex FSJ1. No RF adapters are used, and all N connectors are tightened with a wrench. BNC connectors here would be a compromise; most types provide only 70-80 dB of isolation at 500 MHz. This RF leakage is related to the poorer ground retention of the bayonet lock vs a screw thread. High quality silver N, TNC and gold SMA connectors can provide greater than 100 dB isolation at 500 MHz, if properly installed. That degree of isolation is rarely required, but the input of an unfiltered high gain preamp in a high RF environment is one such place.

DIN 7/16 connectors would be better still. They have the advantages of very low leakage/high isolation and exceptionally low PIM, but are expensive. There are a couple of very nice mini versions of the 50 ohm DIN connector that measures 4.1/9.5 and 4.3/10mm, also have great specs, are somewhat less expensive, and would be an excellent choice for this type of application.

2 cavity filter

In the current repeater configuration, the goal was to just supplement the excellent front-end of the Hytera repeater in a heavy RF milieu with enough gain to overcome the losses of the duplexer, RX cavities and feedline. So currently, a 10 dB type N attenuator is installed between the output of the preamp and the RX input of the repeater. With about 22 dB of preamp gain, this setup yields about 7-8 dB RX gain over the sum of incurred losses. The estimated overall system frontend noise figure should be about 6.3 dB. Derived as 1.6 (feedline) + 2.0 (filter) + 1.2 (duplexer) + 0.5 (preamp) + 1.0 (Hytera RX_FE). This calculation really brings home the NF advantages of a separate RX antenna with the preamp mounted very near the antenna. Unfortunately, this just isn’t practical at most radio sites.

In summary, I have found these preamp modules to be a great bargain. Their construction quality and performance is surprising good even at several times the price. Besides use as a preamp, they can be used for general purpose RF gain blocks over a wide frequency range.

Dave, K7DMK

New repeater is on the air

The new mixed-mode Hytera repeater went live at 15:00 on 2/1/20. As discussed previously, it is a DMR repeater on the Brandmeister network, and an AllStar analog FM repeater on ASL. Our club callsign K7EVR is assigned to the station with a -DR suffix for “digital repeater”. It should be an interesting experimental platform.

Through some generous and gracious friends, and some help with frequency coordination, an excellent site for the repeater has been provided. The antenna is about 1200 feet above the average terrain of the metro Phoenix East Valley. Although the repeater output power is limited to about 40 watts, the anticipated coverage should be excellent except straight South of the location, where a somewhat higher peak blocks propagation.

The site has many other commercial and Amateur repeaters, but the receiver is well protected by a pair of large bandpass cavity filters in addition to the duplexer. The 2 dB loss of the filters plus about 1.6 dB in the feedline is compensated by a low noise preamp based on a SPF5189 MMIC. A dual stage isolator on the TX, and PD526 six cavity duplexer complete the RF setup. All interconnects are done with FSJ1 Superflex, with the exception of the new duplexer harness. There, the old original single shielded RG8 coax was replaced with Belden RG142u and high quality silver plated N connectors.

I was very fortunate to be allowed to use an abandoned DB408 antenna fed by LDF5 Heliax that was present at the site. Adding all losses, about 20W of RF is feeding the antenna.

DC power for the repeater is provided by a home brew N+1 redundant power supply using two 15V 25A Meanwell switchers and Schottky diode isolation. It’s a bit of a overkill; the repeater only draws about 8.5A on TX, but the supplies are PFC corrected and are efficient even at light loads. An external diode isolated DC input with a 5V 5A buck regulator module for the Pi is also provided, but not used in the present configuration. The RasPi used as the AllStar controller is presently powered by another 5V 3A Meanwell supply with a separate AC input, so both the repeater and Pi can be switched on and off independently by an 8 port APC IP power switch. Doing this with external DC power applied would require another way (relay or MOSFET switch) to interrupt the DC power to the repeater for remote reset. An additional DC filter board with an inductive/capacitive Pi configuration provides very clean 12V power to the preamp. A 5V linear regulator and more capacitive filtering is present in the shielded preamp case for 5V Vcc supply to the SPF5189Z.

The site has an emergency generator for AC backup which is augmented with a 600W Magnum Energy inverter-charger unit with it’s internal transfer relay, plus a deep cycle battery that together act as a UPS. This should hold things up until the generator comes on line. The overall system is very efficient compared to many repeater setups. Only about 130W of AC power is consumed during TX and about 30W on RX standby.

The gear is housed in an 18U freestanding fully enclosed steel rack cabinet with front and rear doors. This should provide a degree of additional shielding. Fans in the repeater, power supply, inverter-charger and cabinet will keep things at a reasonable temperature.

Some items that need to be fine-tuned are hang times for both the DMR and analog sides of the system, and auto-disconnect intervals for ASL connections. The RasPi that controls the AllStar functions can be SSHed into on a custom port for editing settings remotely. Additionally, Hytera RDAC is in place as needed to monitor forward power and VSWR, as well as change some, (but not all) codeplug parameters. Required out of band control is provided by Zoiper, a phone app that connects via IAX to the Asterisk AllStar controller. As a last ditch, the APC IP switch can be accessed to turn off and/or reset the Pi, or kill power to the repeater.

Dave K7DMK

K7EVR Digital Amateur Radio