May I add to Don's comment by noting that other benefits to be gained from
the use of a properly designed link coupled tuner (we used to call them
Antenna Matching Units or AMUs) are:
1) This type of tuner being a bandpass circuit helps to reduce the level of
unwanted signals reaching the
Joe,
You may recall that the subject of K3 Mush was discussed on the K3
Contesters Forum earlier this year, and that I suggested that the Group
Delay Variations introduced by the narrower bandwidth roofing filters might
well be the cause of the Mush problem. The recordings of K3's audio sent
Merv,
Does the mush disappear or noticeably decrease if you switch to a much
wider bandwidth roofer (1800 Hz or more), while keeping the DSP's bandwidth
set at 400 Hz or less?
73,
Geoff
LX2AO (aka GM4ESD)
Merv Schweigert wrote on 04/12/2011 at 23:51 +0100:
I start noticing it with more
Ed,
If you switch between different roofing filters, does the sweet spot occur
at the same position of the RF Gain control knob or does it change? Also
does the noise / S meter dip by the same amount regardless of which filter
is in use, if not does the depth of dip increase or decrease when
Pete,
I have not seen a K103 board but when working on a board assembly, most of
my homebrewed ones are populated by SMDs, I always add legs to both sides
of the board. These legs are made from metal spacers - the type which has
a threaded insert at one end and a screw at the other end. If
Larry,
As an older-model ham I have always understood KN to mean just what you
wrote.
Back in the Jurassic when I was first licensed (1946) in a country which was
still quite rare at the time (VQ8) it was not necessary to use KN, because
in my experience most people in those days would not
This and your previous test message both appeared on the List.
73,
Geoff
GM4ESD
- Original Message -
From: Charles chas...@gmail.com
To: elecraft elecraft@mailman.qth.net
Sent: Sunday, July 17, 2011 8:46 PM
Subject: [Elecraft] Test again :-)
I am sending this again - as a further
Jack,
As Bill W4ZV confirmed one of the AGC loops uses a sampling point which is
after the roofing filters but before the DSP and its narrow filter(s).
If the unwanted signals are greatly attenuated by the DSP's filter, but they
are either within the roofing filter's passband or in the roofing
Ron,
I opted to include the KAF2 option when building my K2/100 rather than the
DSP option, and have found that the KAF2 audio filter is a worthwhile
addition to the receiver. FWIW I use a sidetone (beat note) between 350 and
400 Hz when copying CW, and when there is a lot of on frequency QRM
IMHO it is a pity that the KPA500 does not have two output ports (well
isolated), one to connect to an antenna, or a remote ATU, and /or antenna
selector switches - the second output port to connect to a dummy load. I
have been considering various linears capable of 500W - 1KW for use in the
Dave,
The sampling point for the K3's hardware AGC is before the DSP in the signal
path, so unfortunately the DSP filter does not protect the HAGC. The roofing
filters protect the HAGC.
There is another design approach to eliminating AGC pumping problems, but
that is another story :-)
73,
Evening Fred,
If the pipe is OPEN at its ends, would not the current flowing on the
outside of the pipe fold over at the end of the pipe and continue to flow
on the inside skin? The reverse, if you like, of the situation where a coax
feeder is used to feed a dipole / doublet.
73,
Geoff
GM4ESD
Point taken Ian, grey cells in reverse gear here :-)
73,
Geoff
GM4ESD
On July 09, 2011 at 19:17Z, Ian White GM3SEK wrote:
No - HF fields cannot propagate inside an open tube because it behaves
as like a 'waveguide beyond cutoff'. Any HF EM field injected into the
open end will only
Good evening,
On or about 20th July 2011 I shall be leaving the Discussion List prior to
returning to Luxembourg. My present e-mail address will be closed on 24th
July.
73,
Geoff
GM4ESD
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Geoff,
It struck me when the squeal was first mentioned that something might be
arcing over. If I understand correctly the squeal appears only when the
100w TX amp is - which narrows the search if arcing is the cause.
If the 12 KHz squeal is present only when a tic is generated, and if arcing
Vic, I wonder if this is a special skill or an example of some normal human
ability which has not been suffocated at an early age by some poor teaching
process, at school for instance,
In the world of music some of our friends, and many others of course, can
listen to a short work which they
Hi Roger,
I should have twigged that you were using a K3 not a K2, so many thanks to
Bruce N1RX for his comment.
The low power drive solution would be the best option IF you had the time,
provided of course that the IMD contributed by any additional amp if
required in the transverter was
Roger,
A resistive power attenuator adding a db or so loss between the KPA100 and
transverter would allow you to run a little more power from the KPA100, and
you would most likely not have to bypass it during receive. An extra db or
so addded to the transverter's receive IF output (28 MHz)
Roger,
I forgot to mention that my KPA100 switches in at 11 not 13 watts, and is
cleaner at 11 watts (KPA100 In) than at 10 watts (KPA100 out).
73,
Geoff
GM4ESD
- Original Message -
From: Geoffrey Mackenzie-Kennedy gm4...@btinternet.com
To: Roger Dixon li...@irdixon.plus.com
Cc
That is why when flying I always take with me the Manual(s) and Invoice(s)
for any commercial gear in my luggage (and my licence of course), which
helps them to understand if questions are asked :-)
73,
Geoff
GM4ESD
On June 26, 2011at 20:00Z, Phil Kane wrote:
But will they understand what
Doug,
Forgive me for speculating, but it could be interesting to keep your beam
Northwards between now and say July 5. When I lived in VE2 land the VE8 6m
beacon then in the NWT, which only ran a few watts during its early years,
often came romping in with a T9 flutter signal (probably Auroral
Phil,
FWIW the K7IDX 6m beacon runs 5W (TX power out) into a dipole 40ft AGL aimed
E-W, according to a list of beacons which I still have filed on my PC. This
particular list might be out of date.
73,
Geoff
GM4ESD
:
On June 23, 2011 at 18:36 Z, Phil Hystad wrote:
I went looking for your
Hello Kevin,
Trust that all is well with you.
It crossed my mind that if your QTH is off the back of Doug's W7RDP/B Moxon
and you are some distance apart, you might not hear his XG3 beacon because
of the large f/b ratio that is typical of a Moxon. For example one of my
wire 40m beams was a
Not an expert Wayne, but earned my $ calculating path loss at HF up to
microwave during point-to-pont, mobile and relay systems design back then
using the classic 4/3 Earth Radius approach at VHF-Microwave. Unfortunately
my relevant data is packed, but I understand that the path between Kevin
Ed,
An approach which I have used for many years which keeps me sane (?) is to
use a homebrewed box containing rotary switches to select which rig or which
connector of a rig should be connected to the key (or which key), mic,
phones, loudspeaker or whatever. IIRC the generic term for such a
FWIW my favourite method to place a line in a tree makes use of a strong but
light fibreglass pole almost 10m in length, through which runs a drop line.
It does takes longer to place a line than when using a missile system, about
an hour to place a line at 100ft in a tall pine exactly where one
Arie,
I well recall that many experts were predicting similar gloom in the mid
1950's and were proved wrong by the Grandmother of all solar cycles of our
time. Also IMHO our understanding of propagation vs solar activity is still
elementary.
73,
Geoff
GM4ESD
Arie Kleingeld PA3A wrote on
Hi Wayne,
Absolutely!! During my last CW QSO with CA on 15m using my K2/100, he could
still copy me S4 with my Tx power wound down to 5 watts - and this was at
near midnight here when the band here was allegedly dead.
Blood pressure is not a problem for me after 60+ years on the air, cross
Direct Sampling ?
Congratulations!!
73,
Geoff
GM4ESD
On May 19, 2011 6:43 AM, Wayne Burdick wrote:
Totally different architecture than the K3, of course.
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Hello Jeff,
Thank you. When selecting the options offered by Elecraft I chose to receive
a copy of my own posts as a check, and was receiving them until about 36
hours ago.. I looked to see if my List options had been changed, but they
are still as I chose them. Also checked my server's spam
Sorry to be a nuisance.
Geoff
GM4ESD
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Sorry for a me too post, but I agree completely with Mike's comments about
Flexweave. I use the type which is enclosed in a clear UV resistant jacket
because there are many types of birds living here, buzzards, hawks and
smaller types, many of whom like to perch on my wire antennas, and the
jacket
David,
I take your point. Not sure about the use of twin loudspeaker wire for
portable applications. It might be satisfactory when split to make the
antenna element, but could be lossy when used as a feeder, don't know.
73,
Geoff
GM4ESD
On May 16, 2011 at 12:48 PM, CUTTER DAVID
I can
Hi Dale,
K1TD made some measurements back in 1979 on a transmission line made from
dry Zip Cord, and reported that its nominal characteristic impedance at HF
was 105 ohms and its velocity factor was 0.69.
He also measured the loss per 100ft at various frequencies when the line was
terminated by
Jack,
Yes, I have been getting some posts but seemingly not all during the past 24
hours or so. No copies of my own posts.
Your post arrived here 7+ hours after you sent it.
Geoff,
GM4ESD
On May 16, 2011 at 5:47 PM, Jack Regan AE6GC wrote:
I stopped getting posts from the list a few days
Bob,
An interesting idea for a multiband centre-fed antenna was described by
W4JRW in the December 1960 issue of QST, Multiband Antennas using
Decoupling Stubs.
The decoupling stubs that he used are not hung at right angles to the
antenna wire, but are horizontal stubs which results in a tidy
Hello Wes,
The practical question how much non-linearity is acceptable in commercial
HF systems has been debated by some Regulators and their advisers from
industry for years - certainly since the early 1960s in my experience.
In the context of linear amplifiers (or complete transmitters)
Hello Guy,
The short answer to the first part of your question is yes, and the generic
term for such a scheme is Predistortion. A term that has been used to
identify a Predistortion scheme which employ digital techniques is a
Digital Predistortion Linearizer (DPL), and its use can bring about
H, I worked for RCA International (Canada) starting in 1950. Vintage???
Watch your words Ron G
Geoff.
GM4ESD
On April 19, 2011 at 22:56 +0100, Ron D'Eau Claire wrote:
snip
Transmitters will be Henry commercial HF5000D units except for 7050.0
which
will be using a 1950s vintage RCA
). But in almost
all cases the harmonics should not matter.
Alan N1AL
On Fri, 2011-04-15 at 21:30 +0100, Geoffrey Mackenzie-Kennedy wrote:
Hi Alan,
Sorry that I was not here to reply to your previous e-mail.
With respect, filter non-linearity and the level of harmonics coming from
, but normally it would not be an issue.
Alan N1AL
On Fri, 2011-04-15 at 00:37 +0100, Geoffrey Mackenzie-Kennedy wrote:
Hi Al,
You did say normally not an issue, but when making even or odd order
intermodulation measurements on a receiver the use of RF sources whose
harmonic content is significant
Hi Alan,
Sorry that I was not here to reply to your previous e-mail.
With respect, filter non-linearity and the level of harmonics coming from
the signal sources are related in terms of the amplitude of the IMD products
seen at the output of the filter.
If the receiver being tested is perfect
Hi Al,
You did say normally not an issue, but when making even or odd order
intermodulation measurements on a receiver the use of RF sources whose
harmonic content is significant can and usually will lead to invalid
results.
73,
Geoff
GM4ESD
On April 14, 2011 at 22:58 +0100, Alan Bloom
Graham,
As a temporary measure and if there is room, a Thermal Jacket for the
crystal made from a piece of foam packing material might help to reduce
drift.
If the pcb is being heated during transmit, then some other components
(including the pcb itself) in the transverter along with the crystal
Lew, it might be some type of armoured coax. In one type, which IIRC goes by
the name of Armadilo, the coax is encased in a flexible metal tube. When the
flexible tube is covered by a non-metallic jacket, the armour feels like a
single wire wound spiral-wise - and 1/2 turn per inch sounds about
Wayne,
If you have no choice and have to use an antenna in the attic placed close
to the rafters, I would suggest that you do use iinsulators to prevent any
part of the antenna touching a rafter. A fire risk could exist if the
antenna's wire is simply laid on the rafters, especially if using
Julian, I have been using the same type of PSU for my 12V rigs this past
nine years with no problems arising (touch wood).
The usual disclaimers apply.
73,
Geoff
GM4ESD
Julian, G4ILO wrote on April 12, 201 at 17:03 +0100:
I use a Diamond GSV-3000. It is a transformer based supply and has a
Steve,
I have not measured the roll-off of the the K2's bandpass and low pass
filters above 28.8 MHz, and I cannot help you at the moment because I do not
have my test equipment here..
My K2/100's receiver did have a couple of fairly strong birdies near the top
end of ten, which IIRC were due
Indeed, being able to talk to a remote tuner without adding extra control
lines is desireable and can be achieved.
One method, which I used in a 1950s vintage homemade link coupled remote
tuner, makes use of frequency sensitive relays in the remote tuner to
control the tuner's band switching
Eric,
Any good possibility that later on you might also release a kW version which
can be used as a remote ATU?
Am planning for the not too distant future :-)
73,
Geoff
GM4ESD
Eric Swartz wrote on April 07, 2011 at 12:11 AM:
Also, there is a KAT500 in the works for release later this
John,
It is possible to get rid of the K2's birdies, but unfortunately in most
cases it requires some delicate surgery to get rid of each birdie *after*
one has determined why it exists.
I do not have here in Scotland the notes that I made when getting rid of my
K2/100's birdies, so I am not
The new AOR AR-7070, which should go into production this year, will
probably arouse interest among serious SWLs.
From the results of tests made on prototype #2, I understand that this
receiver covers the frequency range 10 kHz - 30 MHz without any gaps, and
good sensitivity across this range.
Hi Alan,
If it came to a vote, I would suggest the use of a full height vertical line
to indicate one's transmitter frequency on any panadapter's spectrum
display, because it is much easier to see than a cursor or a marker or a
couple of little arrows, especially at 3 am. Also when using a
Look at it this way Mike, antenna(s) their feeders and their matching
networks are among the most important parts of any station IMHO, and their
performance will reflect on the on-air performance of any rig including
those produced by Elecraft.
Any discussion about antennas, feeders and
Johnny,
Because you like to build, please do not think that SMDs would be too
difficult for you to handle. I, and no doubt other people, felt the same
concern before starting to use SMDs in homebrew projects. Mounting multipin
SMD ICs is not difficult after some practice.
Tweezers of the
Hi Alan,
I see that there have been some wishes expressed on the List about marking
the frequency of VFO B on the P3.
FWIW the scheme that I used with my old homebrewed panadapter (now retired)
was to mark the frequency of VFO B by a vertical line, which ran from the
top to the bottom of its
My thanks to all who answered my question, and to those who offered
suggestions.
73,
Geoff
GM4ESD
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Could somebody in the UK please tell me how much Import Duty (roughly) one
has to pay if one buys a factory built K3 (100 watt version).
With thanks,
73,
Geoff
GM4ESD
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Hi Vic,
Do you know if any plot of the frequency response K3's APF exists, or any
similar data about its bandwidth at different levels of attenuation, and is
its response peaky?
I might have told you that as part of the ongoing development of my MkII
H-Mode receiver, I have for some time been
My apology to the readers of the List, my e-mail to Vic K2VCO re the K3's
APF should not have been copied to the List.
73,
Geoff
GM4ESD
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Jeff,
I see that Bruce K1FFX has made you aware of Don's (W3FPR) and my comment on
the birdie that Bruce found on 21029 kHz, which also disappears when the
KPA100 is not active. However may I add a little more.
Don speculated that some of the oscillator energy (HV Bias oscillator) could
be
Sorry to take up some bandwidth, but I would appreciate some advice.
Yesterday I received an invitation by e-mail to join a K3 Contesters Group
(Yahoo), but so far I do not know whether or not this Group actually exists.
Could somebody please tell me if it does or does not exist.
Why I should
Some form of Tracking Filter? The challenge would be to teach the filter to
ignore stations who are not calling you :-)
73,
Geoff.
GM4ESD
(Sorry Scott for this duplicate, I did not copy the List when I replied.)
Scott Ellington wrote on Monday, January 31, 2011 at 18:38Z:
Now that we have
Hello Ed,
Well done!! When you can get your K2 up and running, I am certain that you
will find it to be a lot of fun to use and to be a great DX Machine in a
small box!
73,
Geoff
GM4ESD
On Sunday, January 30, 2011 at 08:35Z, Ed KE7HGA wrote:
Well I have officially taken the plunge and
Hi Bruce,
During my Kill Birdie project with my K2/100 #3255, I found that all of
the birdies which disappear if the KPA100 is *not* in use were caused by
the fundamental or a harmonic of the KPA100's HV Bias oscillator mixing with
the fundamental or harmonic of one or more of the other
This may or may not be relevant but low side mixing is used on 15m, 12m and
10m where distortion is heard, and high side mixing on the lower frequency
bands where distortion is not heard.
73,
Geoff
GM4ESD
KA1YMX wrote on Saturday, January 15, 2011 at 9:04 PM:
I am sorry I should have
Evening Jim,
The sad fact about this S meter business is that the SINFO (for CW) and
SINPFEMO (for phone) signal reporting codes were NOT intended to report
absolute signal strengths, but how well a signal was received and heard. I
believe that these codes mutated into the RS and RST reporting
David,
FWIW back in the Stone Age when I had VE callsigns, the input/output
offset of the N.American 2m repeaters was usually 600 kHz. The same offset
was used on 6m, although there were not any local repeaters on 6m at the
time (1950-1974)
73,
Geoff
GM4ESD
David Pratt wrote on Tuesday,
Bob,
Yes I did see your post thanks, but have not yet had the opportunity to
digest your results - but I will.
When you made these measurements, what settings did you use for the 'audio'
Hi-cut and Lo-cut?
My thinking was that I would first look at the SA612's output wile injecting
multiple
Guy, have one (or several) on me for suggesting such a sensible idea :-)
73,
Geoff
GM4ESD
Guy Olinger K2AV
On Sunday, December 12, 2010, at 10:04 AM, Guy Olinger K2AV wrote:
Thank you, Bob.
But you have robbed Geoff of his favorite whipping boy. We will have
to take Geoff out to a pub and
Chen,
It is not really my place to comment on this subject not having a K3 to
measure, but I would like to offer the following thoughts related to the
naughty things you mention.
Whereas measuring the receiver's audio output level vs.the level of a single
signal injected into the antenna input
Hi John,
The NPR test is, I agree, certainly a very useful method to expose IMD
problems, but I think that when used as a *diagnostic* tool the requirements
placed on the notch filter's bandwidth limits this method's versatility -
unless one is prepared to build a set of band reject filters
After the perfect manual has been written in English, a useful next step
IMHO would be to have the manual translated properly into some other
language(s). Expensive yes, but there must be a large number of Elecraft's
customers, and potential customers, who have difficulty with English. A
Hello James,
I use a BHI ANEM Mk II with my K2, and get similar results.
My K2 does not have the Noise Blanker installed nor the DSP, one reason
being that man-made noise at this quiet rural location was never a problem.
But, about two years ago some new neighbours arrived armed with some
Ramiro,
Some very good information about Combiners is published on Jack Smith's
website (Clifton Laboratories), also how to perform IMD tests.
The reason for using a lowpass filter between each generator and the
combiner is to prevent the second harmonic of generator A, for example,
getting
Sorry for being slow to reply.
Oh yes, a coil wound on a core made from Ferrite or Powdered Iron will
be a source of IMD products, the culprit being the core. The level of the
IMD products generated by a core depends on a number of factors, which we
could discuss Off List if you wish.
While I
Ramiro,
Are you using a hybrid to combine the SW carrier and the signal generator's
output before sending them to the K3, and how strong are these two signals
(dbm)?
If you are not using some type of hybrid, could you please let me know how
you are adding these signals.
It is possible that
Thank you. My suspicion is that the IMD is being generated outside of the
radios, but I need to take a closer look at your data.
I see that there are many more IMD products below 990 Hz than above 1000 Hz
in the top half of the waterfall in your CW vs DATA comparison, which could
suggest that
Hi Ramiro,
Unfortunately I do not have a K3 to test because my receivers are
homebrewed.
I still believe that the IMD is generated by something outside of the
radios, one reason being that your results from the K3 and the FT-897 are
not very different. The differences between the AGC-S, AGC-F
Bruce,
I believe that your suspicion is correct. The sticking point in the design
of a high performance up-conversion receiver at this time is the first LO,
whose phase noise must be suitably low and whose cost is acceptable. There
can be problems with VHF Roofing filters, but there are ways
Hi John,
Glad to hear that after all of your bad luck your K2/100 is now working
well.
The strong birdie at 3589 kHz was also present in my K2/100 before I
embarked on a birdie killing project a few years ago, which involved a fair
amout of difficult surgery. This particular birdie is the
With respect Ron, as far as I know the term Roofing Filter was in use many
years before up-conversion superhet receivers with a fist IF at VHF started
to appear in the market. It was at least fifty years ago when I first ran
across the term, which was used to describe the first IF filter used
Bill,
One possibility is that this rogue signal's frequency is not actually at
28.002 MHz, but at some other frequency which coincides with one of the K3
receiver's spurious responses when the K3 is tuned to 28.002 MHz. An example
of one spurious response is the so-called image, which in the
Hello John,
The display of my K2/100 #3255 behaves in similar fashion sometimes, albeit
not often. I was at first concerned that this quirk might be a symptom of
some serious problem, but the results of two tone IMD transmitter tests and
receiver noise floor measurements I made showed no
Hello John,
Here I use a 40m dipole at 70 ft fed with open wire line and a homebrewed
remote antenna tuner as a reference antenna system, and I have not seen any
sign of the problems that you have described when using my K2 barefoot, nor
with the KPA100 in line, on any of the HF bands. But I
Hello Stan,
My K2/100 built in 2003 has the KAF2 option installed, and the KAF2
certainly helps when listening to weak signals.
I do not use the Time Clock, which is part of the KAF2, because it generates
a fairly strong click heard every second.
73,
Geoff
GM4ESD
On Wednesday, September 29,
Pat,
I cannot find the schematic of the HF Packer amp to verify this, but my
suspicion is that the input impedance of the amp might be changing with the
level of drive signal. This change will occur rapidly of course in the case
of SSB.
If this is the case then distortion could result either
Don,
Shortly after I sent my comment earlier today I received an email telling me
that the HF Packer amp was intended to be used for CW (Class C?) not SSB,
and that a modification was necessary for its use as a linear amplifier for
SSB - presumably at least a change in the amplifier's standing
David,
The question remains IMHO whether the data taken from short term on-air
tests over distances which involve ionospheric propagation is useful in the
first place.
Unless the vertical patterns of the A and B antennas are very similar in
shape and lobe take off angle, it can be seen that
Is there any reason why the K3's 2.7 kHz xtal filter normally used during
transmit cannot be replaced by a narrower one, say 2.1 kHz, with the
required adjustment of the carrier frequency?
I am looking for a lightweight rig to take on holiday style DXpeditions,
and the K3 would meet the
Alexey,
Yes, you didn't mention the type of IC socket :-)
Turned Pin sockets aka Machined Pin sockets provide very reliable
connections, but are more expensive than the flat contact type of socket
supplied in many amateur kits which can cause problems.
Turned Pin sockets were used in some
Is and has been working here.
73,
Geoff
GM4ESD
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If you are planning to use closely spaced Tx and Rx antennas, I would
suggest that you consider using a Reed relay or similar fast acting relay
to short circuit the external preamp's input during transmit periods.
Without it there is some risk that during transmit periods the external Rx
David,
Have you measured the strength of the BC carriers in the 6 MHz and 7 MHz
bands after sunset as seen at the shack end of your antenna's feeder when
terminated ?
If of any use to you I could send you some screen captures using a 1600 kHz
span after sunset this evening, with the BC
Brian,
No, I have never used this xtal reference number.
The envelope containing the containing the xtals that came with my K2/100 #
3255 kit in 2003 was marked 7. My guess is that either Elecraft might have
changed their procurement spec for the xtals from time to time, and the
number was
Don,
The main difficulty I believe with down-converting a band to audio, and
then up-converting back to RF to demonstrate the performance of the P3,
could be the spurious products generated during each conversion, which might
reflect badly on the true performance of the P3 if the products
If anyone in the UK is looking for a top cover for the basic K2, please let
me know off-list. This cover has not been used since 2004 and I am not
asking anything for it, but I would like to find it a good home before
closing down at this location.
73,
Geoff
GM4ESD
You should try living with the 40m BC stations heard here and working SSB DX
among them, at peak time their carrier levels can reach a measured +5 to +10
dbm off a 70 ft high dipole. They were supposed to vacate 7100 -71200, but
not all have got the message it seems.
73,
Geoff
GM4ESD
On
Thank you for posting K9OM's comment.
FWIW I have a Perseus and two other receivers which both use H-Mode mixers,
one of which uses a version of PA3AKE's H-Mode front end.
With the Perseus used as a stand alone receiver, I have not experienced this
noise problem in the presence of mild or
Thanks for your explanation, I had missed the point of RLVZ's comment.
A related cause and I am speculating here without doing the calculations,
when the noise is rough and signals are at or under the noise floor might be
the added in-passband low level intermodulation products generated by a
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