|fahnen/3.jpg Amps and Preamps|
Amps and Preamps for Your Station
This script from the Houston AMSAT Net was written by AMSAT Area Coordinator Bruce Paige, KK5DO. Authorization is given for the use of this information over any ham band. Please give credit for the script where credit is due.
We previously talked about the types of radios for use when working satellites and the types of coax to use. Now, this evening we will discuss amps and preamps. Since they play an important part in a satellite station I will attempt to give you some things to look for when selecting that preamp.
If you are thinking about adding an amp to your satellite station, you should consider several things. First is how much output power do you need. Second is how much power does your radio put out. And, third if you are going to have a preamp, how much power will the preamp pass without blowing out the finals in it.
Now, let's consider a radio with a 25 watt output. If you purchase a linear that takes a maximum of 30 watts in, you will be safe and not blow out the linear. If you purchase a linear with a maximum 20 watts in, you will blow the linear finals if you are not careful to not output more than the 20 watts. Also, if the 25 watt output will drive the linear to 100 watts output and you are going to use a preamp, be sure the preamp will pass the 100 watts. Otherwise, you will blow out the finals in the preamp.
I use an rfconcepts linear on 70 cm and it is rated at 30 in and 100 out but my radio outputs 35 and it gives me 120 out. For most of the satellites, this is way too much power but if you are going to work sideband or terrestrial then this is great. On the 2m side, I also use an rfconcepts linear rated at 40 in, 170 out. My radio outputs 45 watts and I get 200 out of the linear.
When I was looking for a preamp, I had to be careful to make sure that if I were working 2m sideband and had the preamp on and then transmitted the full 200 watts that I could easily pass it through the preamp. Several models out there only pass up to 150 watts with coax switching. I found a SSB preamp that would pass 200 watts with coax switching and 750 watts with PTT switching.
The only kicker is that the amps isolate any voltage that the radio passes through the coax and that means that if I have the linear in the circuit, then I cannot use the preamp on coax switching. So when I operate satellite, I have the linear out of the circuit and when I operate 2m sideband, I connect it back up.
When deciding on a preamp, look at the signal gain and the noise figure. Some preamps will give you 20dB signal gain and a noise figure of .8dB. Others might give you 15dB signal gain and a noise figure of 1.2dB. You would like to get the highest signal gain and the lowest noise figure. If you could get a preamp with a signal gain of 20dB and a noise figure of 0dB then that would mean that the signal is improved without the addition of any additional noise. Since this is not possible, you want the noise figure as low as possible. As we spoke in previous sessions, there is a price to pay as the noise figure is reduced and the signal is increased. The larger the signal gain and the lower the noise figure, the more expensive the preamp will be.
If you remember last week when we were talking about coax line losses from the different types of coax? If you were using a coax that has a 2dB loss on a 100 foot run and you added a preamp that gave you 10dB gain, you would be overcoming the loss of the coax and have 8dB effective gain.
Also, if you had a coax with a 3dB loss and were using a 40 watt radio, that would be like cutting your power in half. Now, add a linear that gives you 80 watts out and your effective radiated power would be 40 watts, not counting any loss or gain from the antenna. So, if you could not get the best coax like we spoke about before, you could compensate for the losses by adding an amp or preamp to your station. However, which is more expensive, the $200 preamp and $150 linear or the $300 in coax? So, you see, when we were talking about the different types of coax, it might actually be cheaper to purchase the hardline at $2 or $3 per foot than it is to purchase a preamp or linear to compensate for the coax or even a poor antenna.
There are so many things to consider when setting up your satellite station that you really need to think about every aspect of it carefully before you jump in and make some of your purchases. As you can see, if you bought some coax at $1 per foot and found you were not satisfied, you then would buy a linear and preamp. Now you find that you can do some things that you could not do before but you might eventually decide to run the better coax. You might say that our satellite station is like a home stereo. You always seem to find better speakers, better receivers and better turntables or tape players. At some point you become satisfied and feel you have the best for your money.
Well, now you have a basic idea of some of the components for your station. I went with Flexi 4XL cable that is similar to Belden 9913 except it has a stranded core. This makes it easier to bend and withstand rotation on the rotor. My radio is the ICOM 970H although I used to use a Yaesu 736R. I have an SSB 2m mast mounted preamp that will pass 200 watts on PTT. I have the rfconcepts 4-310 70cm and 2-315 2m linears. The only thing I haven't talked about yet are the antennas and you are just going to have to wait for another segment for that.
You will find people using Mirage, Landwehr, Empire and a varity of other manufactured and homebrewed amps and preamps. It really doesn't matter whose product you purchase as long as the specs meet the minimum specs you require to operate your station.
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