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   Home   Help Search  
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 31 
 on: Today at 01:14:37 PM 
Started by WY7CHY - Last post by W1VT
Common mode current is a big problem with low band receiving antennas.  Most of us have to compromise and use small antennas, which means the signal output isn't very good.  But, as long as the signal to noise ratio improves, the antenna is worth using.  Common mode current typically adds to the noise, which is why you need to keep it away from the feedpoint of your low efficiency receivng antennas.  This may be the most common reason why receiving antennas don't work for some people--they may have common mode noise getting into their antennas.

Zack W1VT

 32 
 on: Today at 01:03:22 PM 
Started by VK5EEE - Last post by N4OI
I tend to call initially, then if no replies - tune around looking for callers.

I am just the opposite... listen around and if no opportunities, call CQ!   Different modes, goals, approaches...  why it is a great hobby!

73   Roll Eyes

 33 
 on: Today at 12:55:54 PM 
Started by G4AON - Last post by AB4ZT
Then there is the claim that it somehow computes ERP.  That's a neat trick without knowing antenna gain and system/feed line losses I really want to know how it does that.

That's what I thought. Plus, they don't show a pic of the working LCD screen, no pic of the back panel, no pic of the inside...seems pretty sketchy to me.

 34 
 on: Today at 12:45:44 PM 
Started by N4RME - Last post by KC4ZGP

Then the Alinco rig I mentioned will fit your bill. I take it you have no HF capability as we speak.

CW rusty? Who's isn't. I have recently returned, dabbled in CW and I'm able to copy at a whole
nine words a minutes.

20 meters phone/this afternoon/evening. Join me won't you.

_ _ ... ... _ _

Kraus


 35 
 on: Today at 12:42:46 PM 
Started by N3ZY - Last post by K9AXN
It is considered bad engineering practice to drive a GG amplifier from a solid-state driver without a tuned input circuit or band-pass matching network in between.

What about all the swamped grid tetrode amps on the market?



 
If grounded-cathode, they are entirely different as the input circuit is not in series with the output.
With a sufficient low plate-grid capacitance, the input circuit can be considered to be completely separated from the output.

"Grid-swamping" can be a convenient means to avoid neutralisation in tetrode amplifiers.


The SS amps have extensive low pass filtering.  Are there any actual 2nd harmonic measurements /comparisons between the SS and tube amps using a Spectrum analyzer connected to the cathode of a GG amp??  How well do the low pass filters in the SS radios work?  Does anyone have a report or evidence of a GG amp oscillating at the tuned/operational frequency --- neutralized or not???   

 36 
 on: Today at 12:41:07 PM 
Started by WY7CHY - Last post by WB6BYU
Quote from: N8CMQ

When you saw this problem, did you happen to use different coax lengths for the analyzer and radio?



It can happen in either situation when the rig is grounded or otherwise connected to other wiring.
I've seen the SWR change when I plug a set of headphones into a battery-powered QRP rig.  A friend
found that plugging a coax into an unused port on an antenna switch threw off the SWR.

As KH6AQ pointed out, one of the antenna wires is the outside of the coax shield and anything else
connected to it
.  Any changes to that means you are measuring a different antenna.  Even using
the same piece of coax coiled up on the roof vs. stretched down to the rig can give different
results.


Quote

In my 40+ years of antenna experience, a balun has not been required except for a few occasions where
the antenna was not center fed, or where a mistake was made in measuring the legs of the dipole.



I've put up a lot of antennas without baluns as well, so they aren't absolutely required to get a low
SWR.  But I've often noticed the side effects of not using a balun as they can manifest in quirky behavior.

After all, 40 years ago most of us didn't have a hand-held antenna analyzer.  I did borrow a tube-type
Millen Grid Dip Meter at one point to try tuning my antenna:  I found a resonance on 5.7 MHz and no amount
of trimming could shift it up to 40m or down to 80m.  Turns out that was due to the length of the coax.
Decades later I was testing a field-tunable 75m dipole and found that I could adjust it one way for good
SWR at 3.6 MHz and the other way for 4.0 MHz, until I forgot to retune the radio and discovered that even
the low frequency setting still had a resonance at 4.0 MHz:  that was the coax, radio, and everything
attached to it, rather than due to the antenna itself.  (In practice, the antenna itself was resonant around
3.8 MHz instead.)

My usual estimate is that about 3/4 of the time when a ham puts up a dipole without a balun they don't
notice any problems that they associate with the lack of a balun.  That's not to say that they might not
have extra noise pickup on the shield, or "RF-in-the-shack", oddball directional patterns, erratic SWR
when switching from AC power to a battery, or other sorts of quirky behavior, but they don't associate it
with not using a balun.  In some cases they may even be able to work DX due to vertical polarized radiation
from the coax.  (And there are some antennas sold commercially that rely on radiation from the coax.)  When
my only criteria for antennas were a reasonably low SWR and the ability to make contacts, I didn't know
the difference, either.

But now we understand baluns and common mode current a lot better than we did 40 years ago.  True, I'm
still going to go out tomorrow and string up some dipoles without baluns for Field Day, because that
is how most of my dipoles are set up.  And they'll work OK.  But my two new delta loops have baluns on
them, and I'm experimenting with adding them to some of my dipole center insulators.  Not because I
can't get a low SWR or make contacts without them, but it makes behavior more predictable.



Quote

I suspect there is more to the story, like antenna current flowing on the outside of the coax...



Yes, that's exactly what the problem is:  common mode current on the outside of the coax shield because
it is part of the antenna.  And that's precisely the problem that a balun is intended to solve by eliminating
(or at least reducing) that current.

 37 
 on: Today at 12:40:30 PM 
Started by NI0Z - Last post by NI0Z
I recognize the DUO's shortcomings.  To me none are dealbreakers.  In my experience, I find some real advantages to direct-sampling radios.  Today, I would not consider anything that is not direct-sampling.

Some have found the Elad SW2 software somewhat overwhelming, and that can be a negative.  It took me some time to recognize how feature-rich the software is.  I now can begin to appreciate it.  There are a number of features not found elsewhere.

The multiple USB connectors can offer some distinct advantages though.  Run SW2 on one computer, and PSK31 (or whatever) on another.



David, your videos are outstanding for the DUO and a invaluable resource, and should be viewed in their entirety at least once before purchasing one.  I will be digging into the DUO more in a month or so and hopefully offer a fully objective overview.  Some SDRs I start off not liking and end up really liking while others are just the opposite, exciting and then not so likable.  I am sure it will be much more interesting when I get fully into it.

73
NI0Z

 38 
 on: Today at 12:38:25 PM 
Started by W5DXP - Last post by KC4ZGP

DXP,

Now you've done it. I'm going to have to find my KAM-Plus.

Uuugghhh!!! (Lurch).

20 meters phone this afternoon/evening. Join me won't you.

Kraus

 39 
 on: Today at 12:31:31 PM 
Started by G4AON - Last post by AB4ZT

Whoever they are, they certainly don't want to be known since there is no information about who the owner is or where they are located. 

https://businesssearch.sos.ca.gov/Document/RetrievePDF



Well, that did not work...but if you go to CA Sec of State website and search for Heathkit as an LLC, the registration info is available as a pdf.

 40 
 on: Today at 12:29:46 PM 
Started by G4AON - Last post by AB4ZT

Whoever they are, they certainly don't want to be known since there is no information about who the owner is or where they are located. 

https://businesssearch.sos.ca.gov/Document/RetrievePDF


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