Amatuer Radio Blog

UPDATE 8/5/2022

The amplifier and tuner are both here, safely delivered. Unfortunately, I cannot do anything with them until Monday as I am restricted from lifting anything heavier than a loaf of bread with my right arm until this coming Monday. I just had a procedure where they run a catheter through your veins into your heart to check for blockage. It messes up your arm (where they enter the catheter) for a few days afterward. The procedure went well, a little buildup on the right side but nothing to write home about.

Yes it is!

I went ahead and ordered a new linear HF amplifier and a high-power antenna tuner from PALSTAR.

First item up is the LA-1K RF Sensing Dual HF LDMOS 1000 Watt Amplifier

This baby is conservatively rated at 1000 watts though it uses two 600w LDMOS driver FETs. I plan to operate it lightly at 500 to 700 watts as I am going to be running it on 120V ac mains. (I may upgrade the service in the radio room to 240V at some point, but it is not something planned for the near future.)

The LA-1K is good for 160 to 6 meters. It does not require a band switch cable, although it can use one if you wish. It only requires the PTT signal from the transceiver. The amp automatically samples the transceiver (exciter) RF to determine the operating frequency and configure itself correctly. A simple blip of PTT will do. It is also very easy to set the operating band from the front panel.

This amplifier suits my needs: although we would all like to run a full legal limit 1500W linear amplifier, the difference in cost between this amplifier and a higher power one is a lot. I do not own the infrastructure (antenna, tower, etc.) to make effective use of a monster-sized 1500W amplifier at any rate.

My main goal here is simply to get over the top of DX pileups. I (think) I am “ok” at working DX phone pileups barefoot, but it does take time. You really must work your technique to get in. The amp will make it a bit easier to cut through the pile.

I do not plan to run digital modes such as FT8 with this, nor CW with this amp. (I’ve yet to learn morse code). Some people do FT8 with a linear amp, but DX FT8 contacts are heavily dependent on band conditions. You will easily see signal fading (QSB) greater than 10db, and that beneath the band’s noise floor. A signal -10db below the noise floor with another 10db of fading on top of that is extremely dicey. Catching signals way (-15db or more) below the band’s noise floor is possible but requires a lot of attempts and sometimes sheer luck. I just do not feel it economical to run the amp hard in 50% continuous duty cycle for hours on end chasing after contacts with an exceptionally low success rate.

The biggest factor with FT8 is the band conditions – an amp will make YOU a bit louder, but it will not help the fading DX station. Better to turn the amp off and stick with 100W directly from the HF transceiver.

I plan to also use this amp on the 75m phone band but will need to put up a different antenna wire for that. My current end-fed half-wave antenna is tuned for the 80M CW segment and the antenna’s Q there is very tight.

The SWR at 75 is a bit too high (> 3:1) and would cause some heating in the antenna’s balun. This is not an antenna tuner issue: an antenna tuner can correct what your amplifier and radio sees to an acceptable SWR, but it cannot do anything for the balun. It is what it is, as they say.

My antenna balun is sturdy enough to operate barefoot SSB at 100w on 75m with my current tuner correcting the SWR for my IC-7300 transceiver, which I do currently, but not at 1000W input MyAntenna’ sells a 75-10 wire to work with their existing 80-10 balun, so that is what I will do at some point in the future.

Seriously, you need your antenna to be resonant, or damn close to it, 3:1 or less when working with high power and a balun. The reflected standing wave at high SWR (if high enough) can cause arc-over in your tuner (or amp if no tuner). In the case of an end-fed antenna with a high ratio transformer 64:1 (balun), it can cause heating of the transformer’s ferrite material, which will cause a change in permeability and an increase in SWR eventually snowballing into heating past the ferrite material’s curie point. Once the ferrite hits that point, your balun behaves much like a short.

Next up is the tuner -the PALSTAR HF-AUTO.

This beast is a special auto tuner. Instead of clacking relays to switch caps and coils in and out, this uses stepper motors to turn a rotary capacitor and coil, much like a high-end manual tuner. But it is fully automatic, it plays well with the LA-1K amplifier, and that is the plus factor that sold me on it, despite it being significantly more expensive than the alternatives. You can tweak the tuning further if you wish. This unit is rated for 1800w, so it is not likely to be abused by the LA-1K, (but always perform a tune up with the amplifier on bypass/standby).

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