I wouldn't trust a 3-500Z for long life any more, nor would I trust a new 4-400A. Emission life is now unpredictable on anything from China.

No. Not if they are on value and not carbon.

Steve,

There is NOTHING special or magic about the shape of the coupling loop. The area of the coupling loop affects the impedance transformation ratio, and as you compress the sides in the transformation ratio increases. It does not matter if you change the area occupied by making it elongated, square, triangular, or a fractal.

You can also vary the coupling by changing the length of the wire, series or parallel tuning it, or bending it in and out.

The SWR you see is the SWR you get, with the sole exception of adding something that might add significant loss resistance.

As for coupling to things around it, a loop is like any other antenna. Although antennas are often different in nearby induction fields, that does not mean any particular antenna has an special license to be trouble free. A small loop is no more or no less immune than any other antenna to local noise, or to effects of things around the loop.

People tend to think and articles often claim small loops have almost mystical or magical properties, but they follow the same rules as any other antenna. They have electric fields and magnetic fields, and the radiation is electromagnetic. As a matter of fact, a small "magnetic" loop is actually ELECTRIC field dominant at distances between 1/8th wave and a full wave from the antenna.

There isn't any magic, as you are learning.

73 Tom

I have never used a variac ever to bring up any old radio, tube or transistor.

I check the tubes, pull the rectifier out, and run them for a few hours with a very small fuse to bake any moisture out. Then I power them up and look for abnormal current or smoke.

I'd bet money my success rate is just as good or better than people who use a variac. If we really look at the practice of reforming capacitors, we'll find the alleged variac method doesn't actually do anything worthwhile.

Reforming requires operation at or above rated voltage with a large series current limiting resistance, generally for a 15 minutes to an hour maximum. If the cap does not come to normal allowed leakage in that time, it is defective and needs replaced.   Anyone who thinks a cap reforms from  walking it up with a variac from a modest voltage to a voltage some amount less than rated without a current limiting resistor and adequate source voltage is fooling himself.

I think the variac idea is one of those old wive's tales that just get passed along. If a cap is so bad it fails from normal voltage application, it should just be changed anyway. If someone wants to reform it, then they should at least do it the correct way.

73, Tom 

.....and in addition to what Dan says, consider HOW or why any antenna radiates. They all follow the same rules.

Antennas radiate because of current, and it is all about the unopposed ampere-feet of physical distance across space between two points occupied by the current. There is no such thing as a magnetic radiator or electric radiator in real life, all of the useful radiation from any antenna is electromagnetic and comes from current.

When you think of that, you see the problems with a small loop. Exactly opposite any given point in the small loop is another area carrying opposing current. This would completely cancel radiation, except there is physical space involved. The physical space allows phase shift because of the finite speed of light. The phase error is what allows the loop to radiate. This is also why a small loop has a null through the axis, because distance through space is the same to every out-of-phase section.

Since the loop fights itself in every direction, a small loop has very high current for a given radiated power and has a low radiation resistance. 

As a matter of fact a small dipole the same conductor length as a loop has much high radiation resistance and much lower current for the same radiated power.

The advantages of a small loop over a small dipole are ability to tune with a capacitor. A capacitor might have a Q in the ten's of thousands instead of hundreds, and is much easier to vary compared to the inductor required in a small dipole.

While this offsets the small dipole's radiation resistance advantage in most applications, I'd use a small inductor loaded dipole if I wanted an efficient single frequency antenna. A small dipole would not have the ease of frequency agility, even though it could more easily be made efficient.

A small loop is electric field dominant in most of the near field distance, while a small dipole is magnetic field dominant in most of the near field distance.  You can see field ratio of a loop and dipole here:

http://www.w8ji.com/images/emfield.gif

So much for "magnetic" radiators. :-)

73 Tom

I assume you want a cheap feeder that is easy to install.

That would require scrounging around, but down here I can buy 250 foot runs of 1-1/4 inch heliax pulled from cell sites for less than $1 foot with connectors.  That line has lower loss than almost anything, and can be directly buried.

In my experience, most relay isolation problems for HF relays come from the layout and wiring methods.

When the relay is used to BYPASS a power amplifier, a good rule of thumb is the isolation has to be greater than 20 dB more than amplifier gain over the operating range. The isolation has to be for the highest and lowest expected load and source impedances in operation over the range where the amplifier has gain.

If the amplifier has 10 dB gain over a range, isolation has to be >30 dB over the range at any projected load or source impedance combination.


If the relay is used to isolate a receiver or receive system from a transmitters output line, the relay should keep power levels to a safe limit for the receiver under worse case conditions of load impedance and peak power.

As I said, most isolation problems I see are related to poor layouts, like ground path current loops, than relay limited. I've obtained more than 85 dB isolation at HF with open frame relays, while I've seen some layouts have only 35 dB using the very same components.

73 Tom

From time to time all capacitor vendors have bad batches. It happens much more frequently these days then in the past.

You also could have gotten old capacitors that have gone beyond safe shelf life. Things are bargains for reasons we do not know.
 
Projected life depends on capacitor core temperature compared to rated maximum temperature more than anything else.

http://jianghai-europe.com/wp-content/uploads/JIANGHAI_Elcap_Lifetime_-_Estimation_AAL.pdf


No, because parallel reduces temperature rise over a single cap. Of course you have twice as many to potentially fail.

Just buy good parts, and makes sure the equalizers are good and the correct type. I think Collins used carbon resistors  in some amplifiers, and carbon resistors should NEVER be used in HV equalizers. Use low enough resistance of metal type resistor of proper tolerance (or test them for match), don't just use 100K for every application like a west coast nichrome loving amateur tells people. Too much resistance can decrease life, not increase it as claimed.

73 Tom

The hour rating of a capacitor are not the hours a capacitor is expected to live. The hours on the capacitor have to be run through a formula which is a multiplier. Most people won't live long enough to use the hours up on any capacitor.

A  6000 hour 450 volt 85 deg C capacitor operated in cool air at 450 volts and 35 degrees C core temperature has a projected life of 192,000 hours.

Things are not nearly as simple as numbers stamped on the side. You'd get more than twice the projected life out of a 105C part rated at 6000 hours than an 85 C part rated at 10,000 hours when the core is at 35 degrees C.

I wonder how many Hams pick a poorer capacitor because they assume the hours are directly written on the side of the part?

When we all started on 160, the typical antenna and power was a dipole at 30 feet and 100 watts. No one knew much about anything, and LORAN limited power. The Brits were allowed 10 watts plate input power, like most European countries.

Timed CQ's were of great value, or very few QSO's would happen.

CQ's have equal or more value today, because in order to make a contact SOMEONE has to call to get things rolling. If no one calls, no one will answer. Not only that, there no longer is a small DX Window. A DX station can be almost anywhere within a wider range now.

I think the general guideline is when no contacts are being made, someone has to call. It doesn't do much good for 10 people to call CQ, but someone has to. It doesn't do any good if no one calls.

73 Tom

I can't imagine ever paying what a restoration would cost, unless the person doing the restoration was just doing it for love.

I probably have 2-3 thousand dollars of labor, materials, and parts in a Globe Scout 65.

Of course just getting something running is a whole lot different, but a BC348 can be a real PITA to get working. I know, because I have one. A safe power supply took a half day of work and a pile of parts, and now I have a BC348 that won't kill me but still has no BFO.      :-)

When I got it, some idiot had a line operated half wave voltage doubler in it. The thing knocked me across the room. 

The only reason to add a second balun or isolator in the shack is if the feedline has some problem outside someplace, or unless there is a noise source in the shack causing noise to follow the feedline out to the antenna's area where it gets back into the antenna. If that is the case, the isolator belongs at the building entrance, not inside the building.

The important thing to realize is that loops are no different than other antennas, so far as how they behave or following rules.

I have successfully used loops in arrays of loops since the 1970's. As a matter of fact my record breaking QSO with Japan on 160, working Japan from Ohio while LORAN navigation was running, was made using phased loops.

The problems in this certainly are not new to me.

The reason I could obtain excellent directivity is I was willing to trade efficiency and gain for directivity and bandwidth. I was happy to have -20dBi gain as long as the pattern was stable and very clean, because I was only using the loops for receiving.

A loop array can have medium efficiency but severe bandwidth and stabilitly issues, or give up all hopes of any efficiency and have stability and excellent pattern. It won't provide usable gain, bandwidth, F/B, and stability at the same time.   

Having worked through problems in the 70's others are dealing with now, I'm not compelled to experimentally revisit something that has not changed in the past 40 years. My last large conductor loop was made from very large semi-flexible oval waveguide pulled from a broadcast station microwave link, and was tuned with a vacuum cap. That was probably around 1970-1972.

Loops today work exactly the same as they did back then, and still follow all the rules any antenna has to follow no matter what a you-tube video might say.

73 Tom

I think I figured it out. Some of my keyboard keys do something unexpected while posting. When I hit tab, for example, it takes the cursor out of the post reply window, so I cannot insert a tab when posting.

I sure agree with N4NYY. Most damage problems center around packing.

For example.... while some small tubes can ship inside an amplifier if restrained in the sockets, a 3-500Z tube should NEVER be shipped in the socket. 3-500Z tubes, like all larger tubes, have to removed and packed. Ameritron almost never sees a 3-500Z left in an amplifier in shipping live through the trip.

The largest killer are G forces. Let's say you have a 20 pound transformer and the amp gets dropped 20 inches. That's 1 G acceleration for 20 inches. Now if you stopped it perfectly with an optimum cushion in 2 inches, deceleration would be 10 G's. The transformer would weigh 200 pounds. Worse, no packing is optimum and some really suck, so a 2" packing thickness could result in 50 G's or more. Popcorn or newspaper barely do anything.

This is why the packing has to have several inches of resilent cushion of proper density, or the thing inside has to be very stout.

I'd guess almost 100% of large radio gear packed by people to send to other people is improperly packed, and would not survive a ten inch drop. UPS wants things to survive a 36 inch drop, or something close.

Hopefuly he showed them the box, because that can help.

What I always suggest people do is to take the item to a professional shipping store, and make them pack it and approve insuring the item. Then the STORE is liable.