Narrow band antenna.

I remember the pain in my wrists from polishing load coils and such. Yes, it did make a difference and if you slacked off for too long the stuff would actually become hot when exposed to a few hundred watts.

So, I learned to develop enough dexterity in my left hand in order to switch wrists from time to time. Many tubes of Wenol polishing paste have gone into this effort over the years.

--
Regards, Joerg

http://www.analogconsultants.com/

Try choking both what are labeled "RF Out" and "Shield Cable."

73's Richard Clark, KB7QHC

:-) I saw the comment about "narrow-banding" the images. They were perhaps a little more than we needed, but it was nice to have something we could actually see. They did not take very long to download here, but someone with a slow connection may have troubles.

One comment: usually you do not need much voltage gain. It is enough to get power gain with the FETs. That is, the received signal voltage across the gap of the loop, as resonated by the capacitors, should be high enough to be used with a good receiver. The problem is that the impedance is very high there. But that same high impedance makes for easy oscillation. From what you posted, it sounds like maybe you have identified an oscillation. If the AGC voltage is low enough, does the oscillation stop? The amplifier I built used two stages, an FET input stage and a buffer stage, and it had very low voltage gain--I am remembering about 3:1 or only 10dB, and maybe only 1:1 or 0dB including the output transformer, but quite a bit of power gain since it transformed the high loop impedance down to 50 ohms for the feedline.

Also, there should be no need for the RF chokes from the gate-1 to source, if the loop is grounded at the bottom. If the loop is grounded at the bottom, the loop plus the RF chokes will short out the source-to-ground resistor. Maybe there is not a need to raise the source voltage above DC ground potential anyway. Also, it may help to NOT bypass the sources to ground, to allow some negative feedback. That may help stabilize the amplifier.

If other things occur to me, I will post them...

Cheers, Tom

Hunh? WTH? I have had loading coils, tuning coils, etc. go for years without polishing at 1 KW in regular use. An HF transceiver to be sure, but what is the difference here?

I'm move images to ImageShack

Yes. It's stop. my main reason for this antenna is make very narrow antenna for city. I can't receive to my receiver almost nothing by big nose from computers, lamps and other things. But looks like this antenna did not help. Its amplify narrow-band noise. And even add more noise when oscillated.

The amplifier I built used two stages, an FET input

Source is not grounded for DC. For better transistors matching and overcurrent protection.

If the loop is

Sources is not grounded.

Thank. It's good idea. I'll try.

Can you use a cotton strip looped around the conductor and saw it back and forth?

Called "ragging" very common in musical instrument business going back hundreds of years.

ImageShack

?image=dsc9431tv9.jpg

short out the

Yes, unfortunately noise that is generated more than one or two wavelengths away from your antenna will be almost entirely electromagnetic by the time it reaches your antenna. Antennas do not differentiate between "electrically generated" and "magnetically generated" noise, when you are far enough that the electromagnetic field dominates over any near-field electric or magnetic field. The balanced small loop is good for rejecting electric-field noise only if (1) the noise is generated close to the antenna and (2) the antenna is close to the ground (so the electric field is guaranteed to be nearly vertical) -- -- where "close" means relative to a wavelength. So the small balanced loop is especially good for LF and VLF work.

Perhaps someone else will have suggestions about what else you might try.

Cheers, Tom

I did not hear nothing about electrically or magnetically photons. It's just photons.

Yes. Computer, lamps etc close to antenna.

15 floor of 16-floor building. But I think that in this case "ground" are building walls.

my reason was make narrow-band antenna. For reject all out of band noise.

That is a very worthwhile objective. All that noise adds to the intermod noise and the sooner you can stop it, the better.

In my youth I recall calling CQ on 42 mhz for a couple of weeks. Fortunately, a very sharp yagi and very low power saved me from a violation.

John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough.

:-) Oh, no, not photons again! When you are near to a source -- to a transmitting antenna or to a computer radiating noise -- the fields in general have not developed fully into electromagnetic waves -- photons if you wish. It is quite usual that, close to the source, either the electric or the magnetic field will dominate. Often from noise sources, the near field is predominantly electric, and a properly done loop antenna will reject that, responding only to the, um, photons.

There is a hint here: it is common that tall buildings incorporate a lot of steel, and that will likely act as a shield. I hope this antenna is not mounted inside!

A reasonable thing to do, though a good receiver with a low-distortion and fairly narrow-band front end should not have trouble with out-of- band signals (noise). Do you have a quantitative measure of just how strong this out of band noise is? I'd personally much rather use a preselection filter separate from the antenna, and close to my operating position, to reject out-of-band signals. Even though the antenna you have described has very high Q, I believe I could do better with a two or three resonator filter running at lower Q, since the slope of the attenuation versus frequency is much greater. Unless there was some especially strong signal in the band, I would at least consider a fixed-tuned bandpass filter that covered my band of interest, assuming that band is fairly narrow such as 7.0-7.1MHz.

Can you tell that you are getting the expected antenna bandwidth, about 3kHz at the 3dB points at 7MHz?

If the amplifier at the antenna has a tendency to oscillate, it very likely also has poor intermodulation performance. Be careful that it doesn't destroy the benefits you are trying to obtain.

Cheers, Tom

It's not mounted at all. But for tests I'm put this antenna outside.

Not. Just not received.

I'd personally much rather use a

I will receive QRSS at all. And I think that it would be best way is using narrow-band antenna -> filter -> synchronous detector.

I'm just testing. I will purchase RF generator in next week and test. Now I have only self-oscillation frequency.

Antenna looks like working. I'm receiving a lots of Morse signals at

7.000 - 7050 Mhz. But I cant recognize any voice signal.

This is receiving signal. Looks like narrow-band enough. This is not self oscillation. In self oscillation voltage a few volts.

This is schematics. I'm not sure that I'm correct use gual gate transistors.

I'm not sure that using shielded cable and ferrite chocks is good idea.

np0 caps.

COOL! All worked!

No. Out of band. I'm now add varicaps and all working! Antenna really very narrow.

It's 7Mhz. quarter wave is 10 miters.

I'm use choking: between amplifier and antenna between transformer and coax cable. For amplifier power wires and gain control.

Side Band?

Not enough choking and needs to be repeated a quarter wave down the choked wire(s).

73's Richard Clark, KB7QHC

Then again at the far end of the wire(s).

I've observed that, and I have observed it is not enough from your photo - if you still have self-oscillation. Your pictures do not reveal any choking of the RF Out cable.

As for the diagonal arm for "ground." This is fine insofar as it being placed in the electrical middle of the antenna loop (a ground), but all this rat's nest of wiring throws the concept of balance out the window. I see in earlier pictures where you clean that up with careful routing, but it seems much too busy. This leads me into the comments following:

On another note, the AGC seems overly elaborate, especially when all that wire could be introducing the self-oscillation. Further, wiring in power seems another invitation to problems when a 9V battery would solve that too. Local power would discard the need for the ground coming from the loop's perimeter, eliminate unnecessary AGC, reduce the complexity of choking, lower gain (it obviously has too much), and give you only one coax coming from the antenna.

You need to solve the self-oscillation through removing complexity. When you accomplish that, THEN that is the time to add it back in, one thing at a time. You will probably discover all those features and design gimmicks are not worth it.

73's Richard Clark, KB7QHC

It would seem no further advice is necessary, much less my last bit of wisdom on the topic.

It would help if you were to elaborate as to what actually killed the oscillation.

73's Richard Clark, KB7QHC

It's inside. Nearby BNC socket.

I think that some disbalance should compensate differencial amplifier on transistors.

Yes. But FETs draw more that 10ma each.

Cable length is not problem. I'm living in apartment. I can put antenna outside the window. But not on the roof.

I can make power supply over coax cable. I can put Atmega8 (en example) to amplifier and add DACs for operate varicaps, AGC. I can add rectifier and filter for detect self-oscillation and automatics reduce AGC. But it's not necessary.