has anyone any ideas or comments about mast-head pre amplifiers for 6 metres. Do they work? What are the pros and cons of these devices?
73
Dale VK4DMC/VK4SIX
Pre amplifiers
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VK4DMC
Pre amplifiers
G'day,
has anyone any ideas or comments about mast-head pre amplifiers for 6 metres. Do they work? What are the pros and cons of these devices?
73
Dale VK4DMC/VK4SIX
has anyone any ideas or comments about mast-head pre amplifiers for 6 metres. Do they work? What are the pros and cons of these devices?
73
Dale VK4DMC/VK4SIX
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VK3SIX
Re: Pre amplifiers
Yes I run an MH2.
I originally had it on the RMV tower as my feeder run was 150 feet into RG10fb.
It was great and I could hear VK7RAE day night.
It was up in 1998 and I used it successfully to work the big EU openings of 2001/2002 summer as well as middle east.
But it was fed with a 12dbd gain yagi 110 feet up in a low noise location.
When friends came to use the 40M yagi running illegal power they damaged the MH2 at least twice.
Even when it was turned off and grounded.
I now have it home on a 70 foot tower to an M2 and fed down LDF4/50
I hear VK5RBV 24/7 at 450 km.
The mornings and line noise limit its use on occasions but allow me to hear stuff like A35RK who I worked 4 weeks ahead of the pack.
I hear better than I can work like with VK6 recently who aanswer my 100W cw calls with QRZ? Very odd.
So ups and downs.
I use MH2 as I said with active voltage up which I did not like so I use two feedlines LDF4 staright and RG10fb amped so I can split between them.
I have coax relay up the top and can isolate the preamp in a storm
It uses a U310 mosfet and is 18db.
WHen I switch it on like now I get 3 s units of band noise on a noisy day so its debatable.
If you cant match your ears with good tx skills its pointless.
I hearstuff the other guys dont and they get pissed off.
Like yesterday with the TV from 8000 km away.
Unless you run a serious yagi with long reach amplifying local noise is pointless.
What were you intending to do with it once the E layer dissipates in a few weeks?
Tropo / m/s ? the boundaries are already well known and defined.
Forward scatter?
We are not allowed enough power so you need a kw erp (which I have with 12db gain).
Again any less and boundary layer scatter is ineffective so is m/s which is regarded as being a minimum of 500-1000 watts erp to work (well).
remember you cant work what you cant hear.
The lowest denominator is the far end.
3 cents
I originally had it on the RMV tower as my feeder run was 150 feet into RG10fb.
It was great and I could hear VK7RAE day night.
It was up in 1998 and I used it successfully to work the big EU openings of 2001/2002 summer as well as middle east.
But it was fed with a 12dbd gain yagi 110 feet up in a low noise location.
When friends came to use the 40M yagi running illegal power they damaged the MH2 at least twice.
Even when it was turned off and grounded.
I now have it home on a 70 foot tower to an M2 and fed down LDF4/50
I hear VK5RBV 24/7 at 450 km.
The mornings and line noise limit its use on occasions but allow me to hear stuff like A35RK who I worked 4 weeks ahead of the pack.
I hear better than I can work like with VK6 recently who aanswer my 100W cw calls with QRZ? Very odd.
So ups and downs.
I use MH2 as I said with active voltage up which I did not like so I use two feedlines LDF4 staright and RG10fb amped so I can split between them.
I have coax relay up the top and can isolate the preamp in a storm
It uses a U310 mosfet and is 18db.
WHen I switch it on like now I get 3 s units of band noise on a noisy day so its debatable.
If you cant match your ears with good tx skills its pointless.
I hearstuff the other guys dont and they get pissed off.
Like yesterday with the TV from 8000 km away.
Unless you run a serious yagi with long reach amplifying local noise is pointless.
What were you intending to do with it once the E layer dissipates in a few weeks?
Tropo / m/s ? the boundaries are already well known and defined.
Forward scatter?
We are not allowed enough power so you need a kw erp (which I have with 12db gain).
Again any less and boundary layer scatter is ineffective so is m/s which is regarded as being a minimum of 500-1000 watts erp to work (well).
remember you cant work what you cant hear.
The lowest denominator is the far end.
3 cents
Re: Pre amplifiers
Dale,VK4DMC wrote:G'day,
has anyone any ideas or comments about mast-head pre amplifiers for 6 metres. Do they work? What are the pros and cons of these devices?
73
Dale VK4DMC/VK4SIX
A quick test that you can perform that gives a good indication of whether you are likely to improve S/N significantly is to assess the ambient noise relative to the receiver internal noise.
Step 1: measure the receiver audio noise power out on a quiet channel with a shielded 50 ohm termination connected to the receiver input. (I do mean a 50 ohm termination, not an o/c or anything else.)
Step 2: connect the antenna and measure the receiver audio noise power out on a quiet channel. Check that there is absolutely no deflection of the S meter.
If the S meter has no deflection, and the noise output increased substantially (say more than 10dB or 3 x voltage), then the receiver noise is insignificant compared to ambient noise and improvements in receiver noise (using a preamp) are unlikely to result in a significant improvement in S/N ratio.
If the S meter deflects in Step 2, the noise would typically be in excess of 20dB higher on most receivers, again, then the receiver noise is insignificant compared to ambient noise and improvements in receiver noise (using a preamp) are unlikely to result in a significant improvement in S/N ratio.
If your noise measurement suggests there may be S/N value in a preamp, estimating the benefit requires a bit more quantitative analysis... but try the above for starters. Ambient noise varies with time of day, beam heading etc, so it will take a number of observations to get a feel for the range of ambient noise conditions that you are likely to experience.
Owen
Re: Pre amplifiers
Dale,
Just to put some numbers around this...
ITU P.372-8 suggests that median ambient noise figure of galactic noise alone at 50Mhz is about 13dB, and it is pretty hard to avoid galactic noise. That means an ambient noise temperature due to galactic sources alone of about 6000K.
If a good transceiver has a sensitivity of say 0.1uV for 10dB S/N in 2kHz equivalent noise bandwidth, and 1dB of cable loss to the antenna, the receive system has an equivalent noise temperature of about 720K.
This gives a total equivalent noise temperature of about 6720K.
Now if you deployed a pretty hot masthead preamp and got the receiver equivalent noise temperature down to say 100K, for a total equivalent noise temperature of 6100K, you have reduced the total noise by 0.4dB, which represents an improvement in S/N of 0.4dB.
Of course if there was substantial man made noise, the benefit would be even less.
Sure the preamp helps, but is it worth it? Note, a preamp will increase S meter reading.
You asked the question of a masthead preamp. The question of whether you should put a preamp at the masthead or in the shack again can be answered by the same techniques. Whilst masthead location will ALWAYS give better S/N (as the experts will tell you), but when you weigh up other disadvantages against a potentially small improvement (possibly less than 0.1dB), you might choose to locate the preamp in the shack.
To assess the benefit in your own situation, you need to know a little about your own environment.
In my experience, most hams cannot tell you what their ambient noise level is on VHF and above, but lets see if anyone can offer experience with measured ambient noise on 6m.
Owen
PS: A preamp could improve S/N by more or less than predicted above depending on the transceiver's and preamp's susceptibility to IMD, and you can't work that out from spec sheets. For the same reason, a filter in front of your receiver may improve S/N even though it brings loss to the system, indeed, an attenuator in front of the receiver can improve S/N in the case of very poor receivers in a high noise environment.
Just to put some numbers around this...
ITU P.372-8 suggests that median ambient noise figure of galactic noise alone at 50Mhz is about 13dB, and it is pretty hard to avoid galactic noise. That means an ambient noise temperature due to galactic sources alone of about 6000K.
If a good transceiver has a sensitivity of say 0.1uV for 10dB S/N in 2kHz equivalent noise bandwidth, and 1dB of cable loss to the antenna, the receive system has an equivalent noise temperature of about 720K.
This gives a total equivalent noise temperature of about 6720K.
Now if you deployed a pretty hot masthead preamp and got the receiver equivalent noise temperature down to say 100K, for a total equivalent noise temperature of 6100K, you have reduced the total noise by 0.4dB, which represents an improvement in S/N of 0.4dB.
Of course if there was substantial man made noise, the benefit would be even less.
Sure the preamp helps, but is it worth it? Note, a preamp will increase S meter reading.
You asked the question of a masthead preamp. The question of whether you should put a preamp at the masthead or in the shack again can be answered by the same techniques. Whilst masthead location will ALWAYS give better S/N (as the experts will tell you), but when you weigh up other disadvantages against a potentially small improvement (possibly less than 0.1dB), you might choose to locate the preamp in the shack.
To assess the benefit in your own situation, you need to know a little about your own environment.
In my experience, most hams cannot tell you what their ambient noise level is on VHF and above, but lets see if anyone can offer experience with measured ambient noise on 6m.
Owen
PS: A preamp could improve S/N by more or less than predicted above depending on the transceiver's and preamp's susceptibility to IMD, and you can't work that out from spec sheets. For the same reason, a filter in front of your receiver may improve S/N even though it brings loss to the system, indeed, an attenuator in front of the receiver can improve S/N in the case of very poor receivers in a high noise environment.
Re: Pre amplifiers
I did a quick single measurement at my location today, and on the quietest channel on 6m that I could find, the noise output of the receiver was 300mV whereas with a dummy load it was 85mV.VK1OD wrote:Dale,
Just to put some numbers around this...
ITU P.372-8 suggests that median ambient noise figure of galactic noise alone at 50Mhz is about 13dB, and it is pretty hard to avoid galactic noise. That means an ambient noise temperature due to galactic sources alone of about 6000K.
Using the specification sensitivity of the IC7000, the noise figure is 5.6dB, and the observation above suggests that the ambient noise temperature is about 15,000K.
In that scenario, a preamp might improve S/N, but by an insignificant amount (around 0.25dB ignoring effects of changed IMD).
Owen
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VK4APG
Re: Pre amplifiers
Wouldn't the short answer be: unless you live way out of town away from the usual noise sources, No benefit to be gained.
Spend the money on the antenna and the best coax you can afford.
Spend the money on the antenna and the best coax you can afford.
Re: Pre amplifiers
I know that short answers are appealing to some, but then explanations appeal to others.vk4apg wrote:Wouldn't the short answer be: unless you live way out of town away from the usual noise sources, No benefit to be gained.
Spend the money on the antenna and the best coax you can afford.
The worked example I gave using 6000K as the ambient noise was based on expected galactic noise.
Living 'way out of town' will not exclude galactic noise.
So, in short, I question whether a masthead preamp in front of a decent lineup would improve S/N by more than a few tenths of a dB at any location, and my reasons are set out in the earlier responses.
Owen