Preamp position on 2M terrestrial paths...

[VK2XV]

OK - firstly I am no expert at what I am about to discuss...

I have dappled in radio astronomy and so I am at least familiar with the concept of antenna temperature - but familiar only...

OK - caveats done.

Note that this is a discussion about a specific set of conditions -mine - I am NOT challenging the science as applied to other setups more impressive than mine...

I see a lot of opinions about masthead preamps versus shack preamps. I am interested in the advantages of one over the other in a specific case - 2M SSB terrestrial (0 degrees elevation). In particular for my woeful setup where I have a 12dBd antenna @ 5m fed with 25m of RG-213. All my experience and research from radio astronomy gives me an antenna temperature around the 1000K mark for 0 degrees elevation (EME is another ball game not discussed here). Analysing this using typical LNA and receiver NFs only gives me less than about 0.5dB improvement in sensitivity with masthead over shack positions. As I understand it this is equivalent to 0.5dB loss in MDS. Given the environment of my setup this seems insignificant - am I wrong ?

Of more importance, I think, is that my FT-847 registers barely 2dB improvement in MDS @ 144MHz between its internal preamp on and off. Given the widely accepted NF without preamp for the FT-847 being of the order of 10dB, the internal preamp seems to only to improve it to 8dB or so. I think a good preamp (<1dB) at the receiver would only gain another 2dB (NF~= 6dB) at the receiver. This small differenc is because of the high antenna temperature. If the antenna temperature was 100K instead of 1000K then much more improvement would be gained by utilising low-noise masthead preamps.

In fact my main concern with using a 25m run of RG-213 is not receive performance, but the drop of 2dB in radiated power due to cable loss. In my setup it makes more sense to have a masthead PA...

I am ready with my flak jacket on and stand ready to be corrected...

Cheers

Steve VK2XV/VK2ZTO

[VK3QI]

Steve,

Just to clarify, are you talking about a <1dBNF masthead preamp versus a <1db NF preamp in the shack ahead of your radio OR

a <1dBNF masthead preamp versus an inbuilt preamp in your radio?

There are significant differences in (a) signal handlng ability and (b) switching issues between the two options that also need to be factored into your thinking.


As far as the noise temp of you location @ 5 metres, don't forget that often those weak signals don't necessarily come in at 0 degrees elevation - they could be considerably higher when ducting is involved.


Cheers

Peter VK3QI

[VK4TI]

Replacing the RG213 with a quality Lmr400 with decent plugs (or similar) seems a sensible thing to do ?

[VK1JA]

My (limited) understanding is that a preamp is used to over come (compensate for, to a degree) cable losses.

Mount the pre amp at the receiver and you'll be amplifying the already degraded rx signal along with all the noise that the coax has picked along the way to the shack.

Mount the preamp as close to the antenna as possible.

You may need to play with the combination of mast mounted pre amp on/radio pre amp on/radio rx attenuator/reduced rf gain on the radio to get the best results, cause there's a good chance you'll overload the front end of the radio if you have both pre amps on (mast mounted and radio).

Edit - as mentioned before - upgrading your feed line would be worth while - 25m is a long length at 2m - lmr400 or even ldf450 if you can afford it.

[VK2XV]

Just to clarify, are you talking about a <1dBNF masthead preamp versus a <1db NF preamp in the shack ahead of your radio

Hi Peter

Yes.

The addition bit about internal preamps was just to further illustrate the 'dampening' effect of high antenna temperature on 'expected' improvements.

I also forgot to mention that the high antenna temperature allows (IMO) the insertion of a filter before the LNA without significant penalty whilst reducing IMD birdies and noise.

Cheers

Steve

[VK2XV]

My (limited) understanding is that a preamp is used to over come (compensate for, to a degree) cable losses.

Jayson,

Yes - I do understand the principle. Thanks.

However, the point I was trying to make is that apart from the theory (or rule of thumb) it is useful to actually run the numbers for your own setup to quantify what you are gaining. For me, after running the numbers, the extra effort to provision (and maintain) a masthead preamp seems not to be worth it.

As I said, the principle I understand (I believe), it is the numbers which I was seeking to be challenged.

Cheers

Steve

[VK3ZAZ]

FWIW

Whatever works for you..
You are attempting to overcome feedline losses basically.

http://www.w4rp.com/ref/coax.html

if you have good feedline cables you don't need preamps.
If you have high loss feedlines you could consider a mast head BUT....

if you have high line noise ditto self defeating.

For various reasons including rfi we don't use preamps at work we use minimum for receive RBR aerials of LDF5/50
For runs up to 50 metres.
People will argue the use of preamps at 6M
They are self defeating arguments.

History

1993 I used a LUNAR PA50 into a low phase noise TS670 on 6M and set the 6M eme record 21 years ago from an M2 12db yagi.

When I came back on 2M in 2011 I used the AG25 matching preamps for my 891 and in 2013 set the new 2m record VK3 to ZL1.

But again the prevailing conditions and operator ability and mode all come into it.
And some degree of luck.

My current Preamp at home (VK3OT) is a MINI kits EME173A2

http://www.minikits.com.au/eme173.htm


For all my results the antenna are way up high agl more than you will possibly achieve in a suburban area and located in very low noise areas.
my feeder was RG10FB (which is all I could afford on such a long run up my 37M tower).
Its still only LDF4/50 even though I could use LDF5 offcuts but choose the easily handled runs..

Both antenna were M2 9 el yagis and that is another story.

if you see my wspr log you will see continuous loggings at the theoretical threshold of hearing,
I don't bother to run wspr much every time I turn it on because I hear everything on air 2M within 500k all the time
I notice so does Peter VK5PJ who I think also uses a preamp.
And his wspr logs are remarkable way out 500-800-1200 km unenhanced


Other preamp designs are by Advanced receiver Research VD and VDG series again a tad under $100 US
I was put onto these ten years ago by VK5PJ.

http://www.advancedreceiver.com/


My current preamps are eastern Microwave LNxxLNA units not cheap over $100 US each.
http://fwfvq.tehfm.servertrust.com/default.asp


2 cents..opinions everyone has one..

[ZL2FAE]

Please excuse my ignorance but I thought that 25mt run of of RG213 had a loss of approx 2.5db at 2Mt.

When you add that to say a 1db NF preamp at the rig that means you add the two figures together to get the effective preamp/coax Noise figure which would be 3.5db (add relay and connector losses of .5db) and that rounds up to 4db.

Mast head mounting (with the same relay and connector losses would give an effective NF of just over 1.5db (needs to be put into the overall NF calculation of the entire setup). (I am told cable loss after the preamp has some bearing on NF but a very diminished effect. There is a calcualtion out there for that somewhere I am sure.) Natrually this option comes with the need for high isolation & high power relays at the Ant. I use a combo of relays (transfer relay) to isolate and High Power relay to path the TX. I also use 7/8" Heliax, in fact I recomend that as the first option before purchasing a preamp and relays.

Now if the preamp had a NF of .5db then the effective NF of the preamp cable etc would be a little over 1db. Now that is talking.... Terestrial (mainly QRM) noise will likely kill these gains to a large degree but JT65 etc can dig signals below the audiable noise floor.

[VK2XV]

Please excuse my ignorance but I thought that 25mt run of of RG213 had a loss of approx 2.5db at 2Mt.

I used 2dB loss. When I run the numbers using 1000K antenna temperature (which doesn't drop much from 0 degrees elevation to about 4 degrees elevation at 144MHz), I only get a loss of about 0.5dB in G/T.

Antenna temperature, as far as I understand, is part of system noise. An LNA at the antenna doesn't lower antenna temperature. So the NF of the LNA is swamped by the antenna temperature giving a system NF much higher than the preamp.

Try and run the numbers for G/T on a system which has a LNA at the antenna versus the LNA at the other end of a 25m length of RG213 (say 2dB loss). Assume Ta = 1000K; LNA NF=1dB/Gain=20dB; antenna gain 15dBd and receiver NF= 10dB. Let me know what the difference in G/T you get.

Cheers

Steve

[VK5ZD]

I suggest reading Owen's comments in this thread:
http://www.vklogger.com/forum/viewtopic ... 11&t=12171

[VK2XV]

I suggest reading Owen's comments in this thread:
http://www.vklogger.com/forum/viewtopic ... 11&t=12171

Aha - thanks Iain. I see my results are consistent with Owen's in broad terms of numbers. Also Owen's comments about actually running the numbers strike a chord with me. I like his ROT reference

He uses G/T as a figure of merit which I am most comfortable with (it is used by radio astronomers for a figure of merit for their systems). Most amateur analyses seem to leave out antenna temperature which is central to RA system analysis. I am puzzled by this omission because it is system noise performance which is the key.

Using G/T, IMO, is a better way to find out what is important in your system. One of the things that you notice, if you do G/T calcs, is that with a Ta = 1000K, a lossy cable between the antenna and the LNA LOWERS both the system temperature and system NF which might to some seem an improvement! However, the G/T drops which accurately shows the loss in sensitivity.

Cheers

Steve

[VK3DXE]

Aha - thanks Iain. I see my results are consistent with Owen's in broad terms of numbers. Also Owen's comments about actually running the numbers strike a chord with me. I like his ROT reference

He uses G/T as a figure of merit which I am most comfortable with (it is used by radio astronomers for a figure of merit for their systems). Most amateur analyses seem to leave out antenna temperature which is central to RA system analysis. I am puzzled by this omission because it is system noise performance which is the key.

Using G/T, IMO, is a better way to find out what is important in your system. One of the things that you notice, if you do G/T calcs, is that with a Ta = 1000K, a lossy cable between the antenna and the LNA LOWERS both the system temperature and system NF which might to some seem an improvement! However, the G/T drops which accurately shows the loss in sensitivity.

Cheers

Steve

Owen's demonstration shines some very clear light on what, to many, seems all very mystical and magical. .....And then there are those who get confused by the "preamp" in their radios which should really be looked upon as an attenuator (when turned off). His comments re cavities or other filtering before the preamp are particularly important in the contemporary urban RF environment. I run a number of preamps, sometimes in cascade (with the radio front end suitably attentuated) to achieve or test various ends. I have found that a preamp with a narrow bandwidth response can make 3 - 6dB of improvement on 2m in an urban location, especially where there are other high powered services nearby.

Steve, my recommendation for you would be to run a decent preamp, but given your noise environment, I'd focus more on the frequency response characteristics and not get too hung up on outright figures. I would tend to try and reduce the feedline losses as much as possible, but I think you'll find the MOST important improvement you can make is to try and get your antenna up higher - away from anything that might distort the pattern and, if at all possible, put up MORE ALUMINIUM, preferably using a modern low-noise yagi design.

From what you're describing of your current station, more/better antenna will improve what you're able to work more than anything else. As an example, at my old QTH closer to the Melbourne CBD, I was running 6 elements at 10m agl, about 9dBd gain. I was being heard quite strongly (-19db JT65B) off the moon by some of the big guys, but couldn't hear a peep from them. Once I moved to a lower noise yagi with only 3.5dB more gain, AND got another metre or so of height, I was suddenly able to occasionally hear even a single yagi station (with excellent ground gain, of course). I also saw huge improvements on terrestrial paths, with a reliable range of circa 700km on 2m AE. The important thing to note here is that I saw a LOT more improvement than the 3.5dB of antenna gain.

Edit: I forgot to mention, I run my preamps in the shack, although I am spoilt abit and am able to keep feedline losses generally below .9dB

[VK2XV]

Thanks Alan and others.

All understood. I will invest in better coax in due course.

Cheers

Steve VK2XV/VK2ZTO