Tom is right about this thread splintered. It is bugging me to hear several
very common misconceptions going on for years.
1- A preamp is part of the radio, not the antenna. As such, when you add a
preamp, you are changing the RADIO performance, not fixing the antenna gain.
For low bands IP3 is a must, even for radios that claim + 30db because IP3
has a very poor performance on low bands. For example: IC7600, any KW
station on 160m 5 miles away can wipe weak signals on the band due to
reciprocal mixing. You can do a quick check on how bad it is; if you hear a
strong carrier on 1810, 1820 or 1830, you definitely have a problem.
2- Adding a preamp 20 db. gain you drop the IP3 by 20 db, making the radio
almost useless.
3- RX antenna has low gain and most people understand that it offsets the
IP3 problem. The issue is that RX antenna gain changes with frequency, a
FLAG , EWE K9AY have -20 db. gain on 160, but close to minus 0.5 db. gain
on 7 MHz . So, the strong signals on 6MHz 9 MHs 10 to 12 MHz, can cause a
real damage on weak signal reception. A good band pass filter in the front
and another in the back of the preamp is a MUST for most radios. Using only
a good high pass AM BC filter helps half of the needed protection, all
preamps need a good AM BC filter for 160m operation, and only the NORTON
preamps can perform without a band pass filter bit, but it is not enough.
4- Adding to the problem of increasing the gain and reducing the IP3, the
size of feed lines, rotor cables and most of all other cables connected to
your station (like 144 MHz vertical with 120 ft. of coax cable makes a good
vertical for 1.8 MHz) , are near 1/4 wave long on low bands. It means those
wires are indeed vertical or long wires receiving high amount of energy and
feeding your ground connection. All of them must be grounded in several
places to break the 1/4 wave problem and fix the common mode noise issue.
Using a choke near the ground next to the radio can even reduce the noise
floor on 20m. Not taking care of common mode noise can make any preamp
useless.
5- Noise figure is another huge misconception. GAIN does not improve signal
to noise ratio, actually all preamps and any electronic circuit, adds noise
to the output and decrease the signal to noise ratio. THE SNR OR NF IS
FREQUENCY DEPENDENT and the all available equipment starts the measurement
at 10 MHz and above. NF looks good on GHz for several new devices, but, what
about 1.8 MHz? The noise figure on low bands can be as bad as 10 db. NF, in
some cases even more (the MHz devices are only good for cellphones) Most
radios already have 5 db. NF (internal preamp on) and 20 db. NF (internal
preamp off). You will really need a preamp better than 5 db. to improve the
signal to noise ratio, or you are just adding problems to your RX. The W7UIV
preamp with 5db NF is 100% useless for receiving weak signals on top-band,
if you increase the audio gain and decrease the RF gain in your radio, you
will improve the receiving signal to noise ratio better than adding a preamp
with 18db / 5db. noise figure. In this situation there are no signal to
noise improvement at all!. You don?t need it!
What is the difference between increasing the audio gain in 20 db. or adding
a 20 db. preamp with the same radio NF? NONE, just an AGC comfort for strong
signal handle.
Here is what bugs me the most, some statements like "You don't need a good
1 dB NF preamp on low bands", or "You don't need a preamp at all". These are
the most misconception urban legends ever believed.
The NF concept is based on degradation of the signal to noise radio. It
means that, when the power noise is the same as the power of the signal, the
ratio is 3 db. - half noise, half signal. In this case the degradation is
close to 2 db., which is the button line. The SNR needs to be 3db. or better
for you to be able to copy a weak CW signal or 8 to 10 db. for you to copy a
weak SSB signal.
So, where is the antenna noise floor? The antenna noise floor is depended of
the intensity of the signal delivered from the antenna at the input of the
receiver in dBm, it has absolutely nothing to do with the band itself. A
minus 120 dBm signal is the same on 1.8 MHz and 144 MHz . The confusion
starts with the noise on low bands. Several reports, like ITU 372, show
that, in 95% of the time, the atmospheric noise is very high on low band and
very low on HF, but, how about the other 5%? Guess what , in 5% of the time
the atmospheric noise in 160m gets low as 100K (1db NF) on 2 MHz and 5000 K
on 10 MHz (20db NF). As you can see, the concept of equivalent noise
temperature is very complex for even the most experienced engineers, and I
will not cover it here.
When there is atmospheric noise we don?t even turn on the radio, so, 95%
does not mean much because we only use the radio on low bands when the
atmospheric noise is low. For example, on that winter morning after a light
rain the noise on 160m can be as low as -125dbm, this will be lower than the
noise on 80m or 40m. Guess which weak signal may show up on that day,
Africa? VK0? That DX expedition you need for a new one... you bet. The other
95 % of the time, the NF has no SNR impact if the signal is 10db above the
noise floor, including the internal noise. The degradation is approximately
10% for a bad preamp and 1 % for a good preamp, it actually does not mean
anything. The preamp just reduces the damage of the noise problem on the
electronic circuit, it does not increase anything else, 10% reduction is, in
practice, the same as 1% or 0.1%. The gain I sonly important when you
cascade the preamp with the radio, the noise contribution of the radio will
be its NF divided by the gain the preamp in front of the radio. A radio with
a 20 db. NF, with a 20 db. gain preamp in front, will impact the NF of the
system by 20/20 = 1db, so, if your preamp has 2 db. NF on 160m, the system
will have a 3 db. NF. The trick is that 95 % of the time you won't need the
preamp at all, it will only be needed in that 5% or 1% of the time to work
that weak signal from Africa for a new one on 160m.!!
6- Timing is a really huge problem and a sequencer is also a must. Here is a
very simple TX/RX antenna sequencer that I have been using for several years
that works very well. For CW the fix is very simple, use the sequencer
controlling your amplifier and preamp, and use a new design CW electronic
key that delays the dits and dots 30ms or more, 80ms to 100ms is a good
choice to work up to 25 wpm. That simple feature grounds the PTT and waits
all relays to bounce, after all is stable, it will start sending the CW key
to the radio. Also, most new CW circuits has tail delay to wait the CW kick
back. The small NEC AE2 relays or equivalent are fast enough and also very
small. The ideal relay is the G6K with 80 db. isolation at 2MHz
Douglas, my suggestion for you is that: use that out-put BNC after the
internal TR/RX relay, add two relays sequencer like the G3SEK bellow
http://www.ifwtech.co.uk/g3sek/dx-book/sequencer/
Don?t mess with open frame relays not designed for RF, the lack of shield
you kill your RX performance. Use tin steel plated boxes, they are easy to
find on any food can, the aluminum enclosure does not cut magnetic field, it
also does not help to cut noise from switching power supply or PC monitors.
A shield and a good band pass filter will help you a lot .
Based on my experience with EWE FLAG's and WF in the last 10 years, I
recommend you to use no more than 10 db. gain preamp for single EWE, FLAG
and Beverage antennas (all are vertical RX antennas). For dual phased EWE,
FLAGS or vertical WF, 20 db. gain is enough (use the internal preamp only
during very quiet days). A NORTON preamp (K8ZOA) is the only preamp that I
recommend, it has low NF and high IP3. For horizontal loops 20 db. gain is
enough, and for phased horizontal loops, like HWF, a 40 db. gain preamp is
necessary. KD9SV has a new preamp with 40 db. gain and 1dB NF that I
recommend for phased horizontal loops.
Also try to use a delay CW electronic key like K1EL winkey (the new K16 IC
is just US$8) or any new design with delay and tail adjustment. Several
logging programs also have CW machine with delay feature.
My 20 cents.
N4IS
JC
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Douglas
Ruz (CO8DM)
Sent: Sunday, August 30, 2015 8:42 AM
To: Tom W8JI <w8ji@w8ji.com>; Charles Cu nningham
<charlie-cunningham@nc.rr.com>; 'topBand List' <topband@contesting.com>
Subject: Re: Topband: KD9SV-OK1RR relays ???
Tom and all members of the list thanks for your ideas and time !!!!.
I have a plan B:
I added a BNC female connector in the rear panel of my old Yaesu, FT747 (It
is an FT80C, the commercial version with metallic chassis) few months ago.
I was using that BNC to feed an SDR receiver using the same antenna.
If i add a SPDT switch also on the rear panel and RE-WIRE the BNC connection
i can get an RX port in my old radio and can connect an RX antenna...like
modern radios...Switch Pos A (main antenna at SO239)...Pos B (SO239 for TX
and BNC for RX)...i made few mods to my radio, so, i know it very well...
Maybe that will help...of course, i must add then an Front End Saver...still
need a fast relay !!!
Maybe the Plan A with a 5mS relay will be more easy...still need a fast
relay too !!!
73....Douglas, CO8DM
"No creo que haya alguna emoción más intensa para un inventor que ver alguna
de sus creaciones funcionando. Esa emoción hace que uno se olvide de comer,
de dormir, de todo." - Nikola Tesla
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Charles Cu nningham" <charlie-cunningham@nc.rr.com>; "'topBand List'"
<topband@contesting.com>
Sent: Sunday, August 30, 2015 12:27 AM
Subject: Re: Topband: KD9SV-OK1RR relays ???
>> It seems to me that a very fast operating preamp protection circuit
>> could be constructed employing a good fast saturating NPN switching
>> transistor across the antenna path. In receive mode the
>> collector-base junction would have substantial reverse bias and the
>> transistor can be chosen for low collector-base capacitance. With a
>> fast switch like a 2N708 or something similar the switching time
>> will, of course, FAR outperform a relay closure time.
>
>
> This thread might have splintered. I was responding to this:
>
> <<Your FT-747 only has one antenna input. It does not have a second
> <<receiver. It doesn't even have a receive-only antenna input. You
> cannot <<transmit and receive at same time. Why do you need a "front end
saver"?
>
>
> If it is a transceiver without an RX antenna point, the problem is
> adding a receive antenna to a transceiver that does not have a receive
port.
>
> If it is a transceiver with an RX port, the requirement for an
> external "front end saver" and what will work depends on the antennas,
> the power, the transceiver, and the antenna spacing.
>
> A front end saver can be very simple with some radios, more
> complicated, or not needed at all.
>
> An external switch is never easy to do correctly, unless the radio has
> good TX RX switching time sequencing.
>
> I'm unclear what the application is, but a 10 mS relay is really too
> slow for either application. The sequencing issues I pointed out apply
> to both systems.
>
> 73 Tom
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