How long is your feedline? Jim

--- On Mon, 11/3/08, ANTHONY JAPHA [email protected] wrote:

From: ANTHONY JAPHA [email protected] Subject: [amsat-bb] Preamp advice, pse To: [email protected] Date: Monday, November 3, 2008, 3:45 PM Bird Lovers, Pse let me have advice on which of the following ARR 2 mtr preamps I should get: P144VDA: 1.0db nf, 15 db gain; or P144VDG: 0.5db nf, 24db gain.

The preamp will be used primarily for AO-7. I operate only portable in a high noise environment here in Manhattan. I doubt the nf difference is at all meaningful in my use. The large difference in gain between the 2 preamps may not be impt. either, that is the real question. My XCVR is the FT-817. Another 817 is used to transmit, running abt 15-20 watts into a small amp. with an Arrow split between 2 booms on 2 tripods only abt. 1 ft. from each other.

So what's the verdict?

Tnx and 73, Tony, N2UN

---

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----- Original Message ----- From: "ANTHONY JAPHA" [email protected] To: [email protected] Sent: Tuesday, November 04, 2008 12:45 AM Subject: [amsat-bb] Preamp advice, pse

Bird Lovers, Pse let me have advice on which of the following ARR 2 mtr preamps I

should get: P144VDA: 1.0db nf, 15 db gain; or P144VDG: 0.5db nf, 24db gain.

The preamp will be used primarily for AO-7. I operate only portable in a high noise environment here in Manhattan. I doubt the nf difference is at all meaningful in my use. The large difference in gain between the 2 preamps may not be impt. either, that is the real question. My XCVR is the FT-817. Another 817 is used to transmit, running abt 15-20 watts into a small amp. with an Arrow split between 2 booms on 2 tripods only abt. 1 ft. from each other.

So what's the verdict?

Tnx and 73, Tony, N2UN

Hi Tony, N2UN

Do you remember our QSO via FO-29 on 21 april 2008 ? You writes to to me

Domenico I thought you might like to see a photo of the station used to make our QSO. It is attached. The Triborough Bridge and Hell Gate Bridge are in the background. The station is on a promenade next to the East River, 2 miles north of the United Nations. 73, Tony

I am looking at the photo of your location and I see that you are living in an urban area in which the man made noise temperature at 145 MHz is at best a minimum of 10.000 kelvin

The sensitivity of a FT-817 at 144 MHz is specified to be 0.25 uV but the manufacturer do not refere for what ratio in dB

Supposing that the sensitivity is 0.25 uV for a S/N ratio of 10 dB then the corresponding computed Noise Figure of a FT-817 is 11 dB at 144 MHz corresponding to a noise temperature of 3360 kelvin

Assuming from the photo that you are using 10 feet only of RG213 to connect the output of your preamplifier to the input of your FT-817 then the loss of 0.25 dB at 145 MHz introduced by the above coax cable can be omitted in our calculation.

If you use a preamplifier with NF= 1 dB and Gain 15 dB in front of your FT-817 then the overall Noise Figure of your receiving system becomes 2 dB corresponding to a noise temperature of 170 kelvin

If you use a preamplifier with NF= 0.5 dB and Gain = 24 dB in front of your FT-817 then the overall Noise Figure of your receiving system becomes 0.7 dB corresponding to a noise temperature of 51 kelvin

The improvement in S/N using a FT-817 with in front of it a preamplifier of NF= 0.5 dB versus a preamplifier with NF = 1 dB in the New York City area with a man made noise of 10.000 kelvin will be only (10.000 + 170) / ( 10.000 + 51) = 0.05 dB i.e. a insignificant improvement.

You cannot see the improvement between preamplifiers because the man made noise of 10.000 kelvin on your antenna predominates but if you go in a country location where the man made noise is only 200 kelvin at 145 MHz then the improvement in S/N using your FT-817 with a 0.5 dB NF preamplifier versus a 1 dB NF preamplifier will rise to (200 + 170) / (200/ 51) = 1.7 dB

In conclusion using preamplifiers with 0.5 dB or 1 dB Noise Figure in front of a FT-817 in the New Yok City area the advantage is basically ZERO but it rises to 1.7 dB if you go in the country area very far from the city.

On the other side:

If you use the FT-817 alone in New York City area versus the FT-817 + the preamplifier with NF= 1 dB then the improvement in S/N will be (10.000 + 3360 ) / ( 10.000 + 170 ) = 1.18 dB

If you use the FT-817 alone in New York City versus the FT-817 + the preamplifier with NF= 0.5 dB then the improvement will be (10.000 +3360) / ( 10.000 + 51) = 1.23 dB

You cannot see too much difference with or not preamplifier because again here the antenna temperature of 10.000 kelvin in New York City predominates and again the improvement with one or the other preamplifier is only 1.23-1.18 = 0.05 dB i.e. inexistent

But if you go in a country area where the man made noise is only 200 kelvin then the improvement in S/N using a FT-817 alone versus a FT-817 + in front of it a 1 dB NF preamplifier will rise to : (200 + 3360) / ( 200 + 170) = 10 dB wich is a great improvement ! !

Identically if in a country area you use a FT-817 alone versus a FT-817 + in front of it a 0.5 dB NF preamplifier then the improvement in S/N will rise to (200 + 3360 ) / ( 200 + 51) = 11. 5 dB

In this situation where the antenna temperature do not predominates then the difference in the improvements 11.5-10 = 1.5 dB is greater than the difference between the Noise Figure of the preamplifiers 1-0.5 = 0.5 dB

In conclusion since using a FT-817 with a 0.5 dB NF preamplifier In the New York City area the improvement in S/N versus the FT-817 alone is only 1.23 dB I suggest you do not use a preamplifier but to use an antenna with a greater gain of at least 3 dB

I hope to contact you via AO-7 wich is easier than our previous QSO via FO-29

73" de

i8CVS Domenico

At 02:45 PM 11/3/2008, ANTHONY JAPHA wrote:

Bird Lovers, Pse let me have advice on which of the following ARR 2 mtr preamps I should get: P144VDA: 1.0db nf, 15 db gain; or P144VDG: 0.5db nf, 24db gain.

The preamp will be used primarily for AO-7. I operate only portable in a high noise environment here in Manhattan. I doubt the nf difference is at all meaningful in my use. The large difference in gain between the 2 preamps may not be impt. either, that is the real question. My XCVR is the FT-817. Another 817 is used to transmit, running abt 15-20 watts into a small amp. with an Arrow split between 2 booms on 2 tripods only abt. 1 ft. from each other.

So what's the verdict?

Tnx and 73, Tony, N2UN

Hard to advise you since I live in rural Alaska....but:

I have used both preamps in a slightly different use (eme). The GasFet P144VDG is really nice to get the NF down and has sufficient gain to be used at the antenna with a long run of RG-58 to the radio. But in the RF contaminated environ of Manhattan the P144VDA may work as well (or better due to the high noise floor).

I use the P144VDA as my second preamp in my 2m-eme system to increase the gain of my mgf-1801 preamp since it has 16.8 dB gain and I need a minimum of 22 dB gain before the FT-847. The result is = 16.8 + 15 - 1.8 - 0.5 = 29.5 dB. The last two items are my Rx line loss of 150-feet of LMR-400 and line filter loss.

The P144VDA uses a 3SK48 MOSFet which is quite tolerant of high levels of RF and that makes it a fine second preamp.

So I would say if your line loss were under 2-dB the P144VDA will be OK. The P144VDG will work great if it is not subject to high out of band RFI.

*********************************************************** 73, Ed - KL7UW BP40iq, 6m - 3cm 144-EME: FT-847, mgf-1801, 4x-xp20, 185w 1296-EME: DEMI-Xvtr, 0.30 dBNF, 4.9m dish, 60W http://www.kl7uw.com AK VHF-Up Group NA Rep. for DUBUS: [email protected] ***********************************************************