So, I'm thinking about getting a pre-amp for 2m and was wondering if it was at all possible to have it after the large run of coax so I could use it on two antennas so it would be.

Radio -> Pre amp -> switch -> 2 runs of coax to beam & vertical.

instead of

Raadio -> switch - 1 run of coax to pre amp then to beam / 1 run of coax to vertical

is this possible or am I being crazy?

Will I be all ears no mouth if I've got the ears to hear people but only 50w to get out?

for 40m around the UK i get good reports on about 1w with my mega efficient loop but I fear that on 2m with all the big boys using 400w I'll be able to hear people but they wont hear me.

Chris,

I'm sure Tony will want to contribute, but you lose much of the advantage of the pre-amp if you place it at the shack end of the coax run. The loss in the coax adds directly to the noise figure of the amp to set the noise figure of the overall system.

73,
Steve G3TXQ

Spot on Steve. Its basically a waste of cash Chris. The meter may go further over, but everything will be amplified, noise included. Save your money towards a masthead, mounted close to the beam as is practical and you will hear the benefit.

On the otherhand if you live in an noisy area a preamp may be wasted in any case. It depends on what you are trying to achieve. Also another consideration is the RX front end. Amplifiying at the mast head may create intermod problems if you have strong VHF signals close by and the RX front end is not up to the job.

What radio are you planning on using it with?

Ideally, the pre-amp gain would just be sufficient to overcome the coax loss and establish the amp noise as the dominant noise in the system. Any more gain is unnecessary and, as has been said, can lead to overload of the radio. I've even seen an attenuator inserted between the coax and the radio as a way of coping with excess gain from a pre-amp!

73,
Steve G3TXQ

If you have any noise level showing up on the S meter at all, a pre-amp is a complete and utter waste of time as you don't gain anything other than hearing the same noise at a louder level. You are better actually using attenuation to improve the signal to noise ratio.

Also if the pre-amp internal noise is higher than the noise floor of the radio you reduce the signal to noise ratio of the radio.

Not necessarily true if the pre-amp is at the antenna end of the coax as recommended!

Steve G3TXQ

Not necessarily true as it's possible the noise is induced into the cable.
Also good tuned ccts in the preamp can help reduce out of band signal overload.
There are lots of variables and the only certainty is that the only place to have a preamp is at the mashead.
Inserting an attenuator will reduce the signal, and the noise, equally.
The only possible help is if the front end is being overloaded.
On 2M my masthead preamp seems to contribute little, most of the time, then, inexplicably, it makes enough difference to be worth every penny.

Damn it's hard typing on a mobile.

Cheers

Tony

How many do you think are using 400W?
Certainly not me and very few that I know.
One, or two, use 1600W though.

Cheers

Tony

all taken onboard. it is exactly what i expected unfortunately.

as for how many run 400 i guessed a few. either way 50w from an 897 doesn't compete

got to get a bigger beam!

Bigger beam!
Now you're talking.

Look at the UKAC results to see what can be done with low power.

Cheers

Tony

I've got space limits for the beam to be a permenant fixture on the mast. I've got a loop antenna it has to miss when it goes up and currently I'm close with the 1.5m long antenna.

I'll probably just go for something 3m or so long and have a pain in the ass to raise it from a parked position but if it works that much more I don't really give two hoots

I always thought inline pre-amps were a waste of time as well, re s/n ratio etc..

But I do find the pre-amp on my MK1 706 on 2m actually works, it will drag an inaudible signal out of the noise and turn it into one you can hear.

I intended to show a numerical example, and then forgot!

Let's say we have a radio with a Noise Figure of 2dB, and ahead of it we have a length of feedline with a loss of 3dB; the overall system Noise Figure is then 5dB - the feedline loss adds directly to the radio's Noise Figure.

Now let's say we have a rather poor pre-amp with a 3dB Noise Figure - worse than the radio's - and with 10dB of gain. If we place it at the antenna end of the feedline the overall system Noise Figure becomes 3.4dB - a significant 1.6dB improvement, even though the pre-amp has a worse Noise Figure than the radio.

73,
Steve G3TXQ

The problem is that the RX, on most shack in a box radios, tend to be poor.
The internal pre-amp can help but then you often get an overload problem!
The bandwidth of these preamps can be excessively wide so the signal causing the problem can be many MHz away from the required signal.

The only real solution is an amplifier right next to the antenna.

Cheers

Tony

I challenge anyone to say they can notice a 1.6dB difference.