WD5AGO 144 MHz Cavity Preamps
In another effort to improve my small EME station, I decided to upgrade my preamps (LNA’s.) Previously I had been using WA2ODO preamps. I bought 4 of his over the last 3 years and used all four at one time or another but could not see any difference in what I was hearing regardless of which one I used. I have noticed over the years that the signal reports I GIVE are on average much less than I RECEIVE.
21% of the time I GIVE a better report (by an average of 3 dB) more than what I receive. 72% of the time I GET a better report (by an average of 5.2 dB) than what I sent. 7% of the time the reports are equal.
Since my antennas are on top of a hill and line-of-sight to several FM Radio, TV, 2-way, pager, and cell transmitters plus the urban RFI, I was beginning to think that mixing of out-of-band signals or simple overload might be the cause of my poorer than expected receive performance. So, I felt the next step was to upgrade my LNA’s. I needed to get two because I’m installing a pair of XPOL antennas and will need to have a separate LNA on each polarity.
This time I decided to go with cavity LNA’s to help reduce the potential mixing of out of band signals and overloads which could lead to IMDs in my receiver. In looking around for a supplier of such a product, I decided on WD5AGO. Tommy has been building LNA’s for (it seems like) forever! Plus, he has been entering them in various measuring contests at VHF conventions for many years. In fact, his 144 MHz LNA’s took 3 of the top 4 spots in 2018 at the Central States VHF Society Conference. You can view WD5AGO’s advertising Flyer HERE.
As you can see in the photograph at the top of this Post, I purchased a pair (2) of matched 1/4 λ, silver-plated cavities for 144 MHz. They use an ATF-33143 Low Noise Pseudomorphic HEMT (PHEMT) as the active device. The cavity is 1.5-inches in diameter. After he built them, Tommy sent me the following measurements for both matched LNA’s:
22 dBg @ 0.20 dB N/F
S11 = -7 dB
P1 = +10 dBm
These LNA’s are large and heavy. They measure 17-inches in length and weigh 2 pounds (900 grams) apiece! I will definitely need a bigger box to mount them and the associated T/R relays. Plus, I might need some counter-weights to balance out the mass of these puppies!
WD5AGO gave me a great description of the complexities of designing, building and tuning a preamp. It’s a juggling act to tune a preamp because tuning for the best RL and highest gain does not happen at the lowest noise figure point.
The S11 (input return loss) on my LNA’s (-7 dB) falls in the “Good” range of -6 to -9dB. An S11 of -10dB would be fantastic.
On these LNA’s, the gain peak and RL is about 130 to 138 MHz (25 dBg and -11 RL.) To make these LNA’s better, WD5AGO added extra HP filters to make the 140 to 155 MHz range the peak. Noise minimum is at 150 MHz and that places 144 MHz at the best between both.
With a P1 of 10dB these LNA’s offer a lot of rejection to mixing of signals to produce spurs and random signals. With a narrow band antenna array, connected to these high Q matched LNA’s with +10dBm P1 output, I should see little mixing of out of band signals and overloads to produce IMDs in my receiver.
