New... Shack Blog

A few notes about what's happening in the shack. You can contribute! Anywhere from 300 to 400 words, less if you like, about what goes on in your shack. Email it to the Webmaster.

Do You Really need a Preamp?

So, do you really need a pre-amp for vhf weak signal work? Well, the definitive answer is ... maybe. There are three main factors to consider.

  • The signal/noise ratio at the antenna
  • The noise figure of the first device in the receive path. (including the coax)
  • The coax loss between antenna and the first amplifier (as above)

S/N ratio at the antenna.

Let’s face it, if the signal you want is buried in QRM and/or QRN at the antenna, there is no way a preamp will recover it. It is what it is and a preamp amplifies noise and signal without distinction. The answer of course is to get the antenna into as low noise location as possible. If you finish up with a long length of coax, it’s ok. A preamp can fix that!

Front end noise figure

The maximum sensitivity of a receiver is constrained by the noise generated internally, primarily by the first stage. Any signal weaker than this will be masked by the noise. This is termed the noise floor. To be "discernible", any signal should be 3dB above the noise floor.

Noise Floor = Noise figure + 10logBandwidth -174 dB
where 174 is the arbitrary environmental noise floor.

Clearly, any improvement in Noise Figure gives a corresponding improvement in the noise floor, hence the minimum discernible signal (MDS). Most modern transceivers have a noise figure typical 2 - 4 dB. Modern pre-amps, 0.5 to 1 dB., yielding a potential improvement of 1.5 to 3.0 dB in MDS. Having said that, the pre amp built into the FT-8x7 series is about 1.2dB. A misaligned preamp peaked for gain not noise could well have a NF of 2db...

Long runs of coax.

Would you put a 6dB attenuator in front of your 2M receiver? No I didn’t think so! 30metres of RG-59 is just that, and will increase the noise figure and drop the MDS by 6dB! Same, same 65 metres of RG 213.

So a tall tower and long run of coax needs a preamp at the top of the tower. Once the MDS is gone it cannot be recovered. A preamp with 16dB of gain will bring the signal (noise and signal, remember), well above the point of the receiver MDS.

The definitive answer

  • Yes... If you have a very old transceiver (old land mobiles have 8db NF!) or a poor front end noise figure.
  • Yes... If you have a long run, lossy coax or too many connectors (0.4db/pair “UHF” connectors @144MHz).
  • No... In just about every other case.

 

Young Hams and JOTA

The Upcoming JOTA/JOTI has caused me to reflect on young people's engagement in Amateur Radio. The very fact of JOTI... Jamboree On the Internet.. gives us a little clue as why things seem to be going downhill in terms of participation. Ham radio doesn't have a very good image either. Where it used to be the high tech cutting edge hobby, the image is now that of old me with expensive toys discussing there illnesses and operations on HF. Somehow the achievements in moonbounce, meteor scatter and aircraft enhancement haven't made it into view. I wonder why? Is it the nerdy nature of the hams in these fields, is there some social disconnect or have we simply not taken the effort to showcase those facets.

Under the auspices of Steve Munro, VK2FSRM, we are at least seeing a few diversions from the usual HF efforts with some geocaching and construction projects. As cycle 24 fades into obscurity we will need to do even more to maintain even that modicum of interest. I can't see a bunch of kids sitting around waiting for a rare opening while watching their peers carve up the internet! So what can we do?

So, instead of just communication, which kids can do any time by cellphone or internet, we need to give them something different. Meteor scatter would be a good start. It ties up both Ham radio and IT and it's spectacular to watch the pings arriving. Moonbounce may be a bit of a stretch but it shure would set some goals. Best of all, you can see and hear the pings without having to transmit... no license problems!

Then I suppose we could just ignore it all. By the time we shuffle off this mortal coil it won't matter! I'm alright Jack. Pull up the ladder.

Later
Norm VK3XCI

Couple of Things

Propagation
Looks like the bands are dead from top to bottom at the moment. The Geomagnetic field is at Major Storm and doesn't look like abating anytime soon. Time to do a few things in the shack. Check the graphic in the right column.

Two flat batteries
My bride and I took the van down to Psyche bend for Father's Day and stay Sunday night. That way we could both drink irresponsibly... Hi Hi. I took my little handheld and a spare battery so I could listen and call back to the Sunday broadcast. Alas... I got maybe 10 minutes of the broadcast and the HH battery died. No trouble, I have a spare, supposedly. Got abot 2 minutes out of that, and no way of charging them 'out bush'. So this weeks project is a solar/cigarette lighter charger for 7.2 volt li-ion batteries.

Shack noise
I removed or turned off every thing that looks like it may have an invereter or electronic ballast and the QRM dropped o a much more civilised level. Unfortunately there are some things which just can't stay off. Shack computer for one, actually for two! Not only does the power supply generate a heap of hash, the CPU clock puts out a number of birdies which seem to wander up and down the HF bands. Haven't checked 6, 2 or 70 yet, but since it's a 1.8 Ghz clock, I expect to find some there too.

Remember when 10Mhz was a fast clock?

Making the Connection

Well, Following last weeks debacle with battery chargers, it became quite apparent that I needed to standardize the 12V DC connectors in the shack. I seemed to spend more time making up leads and adaptors than I did sorting the original problem. Not only does this become a problem in the shack but also on various cars, motorcycles and the 12V system on the caravan.

So, what to standardize on? I use three different types, the new charger introduced a fourth. See the pics.

I set some criteria.

  • 20 Amps
  • polarized
  • locking
  • Readily available
  • Reasonable price.

The first criterion was the hardest to meet. Seems that most "sliding contact" connectors max out at 15A. The red and black Anderson type are the only ones rated at 30A, but are neither polarised nor locking, although friction and the contact shape do give some security. A  potential deal breaker unfortunately. The large round ceramic "T" type I've used for ages. They're only rated at 15A, not locking and big and bulky. Also appreciable voltage drop across them at any more than 10 A. Rejected! The black bullet type are pretty good... except they’re expensive and only come pre-wired. Out!

I guess that leaves the nylon shell "T" type. They're rated at 15 A too, but there is a 4 pin version. Paralleled up would give 30 A where its needed, and really, only the FT-857 draws that, and only on voice peaks. So that’s where I've settled for the moment.

In truth, the less connectors in your 12V line the better! Each one is a potential source of intermittency, voltage drop, heat and/or fire. I think battery lug to rig connector direct is the ideal, but I like to go portable and that means plugs of some sort.

And then there's fuse holders. What type and where? another potential trouble spot.


Later
Norm. VK3XCI