Ham Radio - The Doublet antenna, QSOs and observations.

And your point is?

 

I think his point is that many hams (radio amateurs) believe that resonant length, coax fed antennas are "the only way" or "the best way" but the rest of the RF world (non-amateur) for the most part use matching networks to transfer/couple loads. Wonder why.

 

Unless you have an adjustable antenna ie: steppir etc your resonant antenna isnt resonant when you move the dial

Can you name all the antennas that have a 50 ohm feedoint that dont have some form of matching network to get it to 50 ohms

 

Use a doublet when running my WW2 SCR-399. Had quiet some success last June in Normandy commemorating the 75th D'Day anniversary.
BC 610 400W on 1615Kc AM mode. Over 80 contacts covering mostly England, France, Morocco, Belgium, The Nederlands. Was heard all the way to Switzerland.Lots of fun.
Best 73s. PIerre in Switzerland HB9FWY.

 

Running a 51' doublet, ends folded, with 27' of window line direct to MFJ-903B on 40/30/20/15 IN THE ATTIC (HOA) with great success! Working the world...

 

How much power? Sounds like a good design for those living in antenna restricted communities. Might also be a good choice for portable operation.

 

Resonant antennas obviously "aren't the only way". Non resonant antennas are compromise antennas that minimize cost and effort at the expense of competitiveness in a crowded contest situation. Which is why the open wire ladder line crowd is not interested in contesting. "We just want to make some contacts" is what I hear from them.

 

I guess you missed my earlier post. this was written by a PROFESSIONAL engineer, not a ham
Here is a link to a website for professional engineers that explains why RESONANT antennas are best. https://www.edn.com/understanding-electromagnetic-fields-and-antenna-radiation-takes-almost-no-math/
I have antennas that have an SWR of less than 1.5:1 and I use a Palstar AT2K with an Alpha 86 at 1500 watts. Because I had a harris RF 103 1 KW (250 pound) amp with a 3-1000Z that regularly blew up the connections to my 43' vertical due to high voltage from high SWR, even with a tuner. high SWR is a fire hazard and lethal shock hazard at high power. As well as less effective and communicating through the ionosphere.

AF4RK

 

I didn’t think I would enjoy contesting, but I have been in four contests in my short career and have done all right considering that I was only able to operate for short stretches. My doublet worked well and I intend to enter and enjoy more contests.

I come across ops using all kinds of antennas and many different types are effective- the idea that one type of antenna is the be all and end all is not true.

 

Most people that I hear on the resonant antenna band wagon are talking about half wave dipoles or 1/4 wave verticals. Anything they can feed with 50 ohm coax that doesn't require a matching device to work with their modern ham radio set.

They seem to forget about antennas such as half wave, 5/8 wave, etc. They have gain over the 1/4 wave and require a matching device to work with 50 ohm coax.

A guy with a ladder line fed doublet can have some gain over the simple coax fed dipole. It doesn't have to be a compromise.

 

A non resonant antenna that is longer than needed could have gain over a cut-to-resonance antenna, as SV2HZF and AG5CK pointed out. I think the blanket statement that non resonant antennas are compromise is false.

The ladder line crowd knows a matching network similar to the one inside your Alpha amplifier (that matches the output impedance to the plate impedance of your tubes) can be used to match the same load to the antenna with low loss open wire transmission line. And more fuel for thought- Hardly ever accused of being lossy inside an alpha, henry, etc amplifier the same matching scheme is now scrutinized for loss and considered compromise when used to couple an antenna instead of the plate of a tube in many discussions. Just a general observation, not directing the last comment towards anyone, as that has not quite come up yet here.

 

You didn't mention which bands(s) you were operating when you "regularly blew up the connections to my 43' vertical", but my guess is they were the bands where a 43' antenna is not an efficient radiator of RF. It had nothing to do with the fact that the antenna wasn't "resonant" or that it presented a high SWR at it's feedpoint. Here's a couple of articles, also written by a PROFESSIONAL engineer:
http://w8ji.com/short_verticals.htm
http://w8ji.com/short_dipoles_and_problems.htm
To summarize: an antenna needs to be at least .4 (four-tenths) of a wavelength long (for a center-fed dipole) to efficiently radiate RF. If the antenna is shorter than .4 wavelength, the rest of the system (balun, feedline, matching network, insulators, etc.) must be able to withstand the excessively high voltages and currents to which they will be subjected due to the power that is NOT radiated due to ANTENNA INEFFICIENCY, not due to the fact that the antenna is not "resonant".

Regarding your link to the article by the PROFESSIONAL engineer, I believe this is the part to which you are referring to support your belief that antennas must be resonant:

"Figure 10 shows a plot of radiated power as a function of antenna length. When the antenna is smaller than a wavelength, the radiated power is roughly proportional to the length. However, for wire lengths near or above a wavelength, the radiated power relates as a slowly increasing and oscillating function. So, why is a length of λ/2 usually chosen for dipole antennas (λ/4 for a monopole)? The “diminishing returns” of the radiated power versus wire length partially explain why dipole antennas' length is usually chosen to be less than a wavelength). The length of λ/2 is chosen because at this wavelength, the antenna is electrically resonant, which makes its electrical impedance purely real, and the radiation pattern is simple (single-lobed) and broad.


Figure 10 Radiated power emitted from a dipole antenna is a function of the antenna length. The source current is 1A."

That excerpt is not critical of antennas that are longer than 1/2 wavlength, it merely illustrates that their efficiency is not much more than if they are a minimum of 1/2 wave long.

Many AM broadcast stations are diplexed into a single antenna or antenna array. Such an antenna can be resonant on only one frequency. The antenna is matched to the non-resonant frequency with very low loss, as long as the antenna is at least close to a minimum of a quarter wavelength (for a ground mounted vertical) long at the lowest frequency. Ditto for FM and TV broadcasters, many of them feed multiple transmitters into the same antenna. Shortwave broadcast stations use distributed feed curtain arrays on multiple frequency bands, using matching networks to tune the antenna to resonance. The DUGA-3 (Russian woodpecker) is another example of a non-resonant antenna that was used on multiple frequency bands. An example of an inefficient antenna is WWVB, the 60 kHz time signal station. It's an electrically short antenna, but it manages to radiate about 70 KW of the 100 KW that is supplied by the transmitters. A "resonant" antenna would be impractical at that frequency.
https://www.nist.gov/pml/time-and-frequency-division/radio-stations/wwvb

 

I read through most of that. I'm not qualified to dispute the physics so I'll assume it's correct but I'm skeptical that it explains why resonant antennas are best.

How do you make that conclusion from that article? It does say that short antennas (much less than a wavelength) don't radiate well but I don't see how it says longer antennas radiate better if they're resonant.

Maybe you're looking at the figure 10 graph but you need to look at it carefully. It shows radiated power by antenna length but with a constant current. The impedance seen by the source would vary by length so therefore the voltage and therefore power would need to vary to maintain the same current. That graph doesn't show radiated power efficiency, i.e., power out for power in.

Maybe you're thinking something about the distinction between storage field and radiation field but that doesn't really add up either. The storage field is near field.

But forget all that and answer this question: If a non-resonant antenna does not radiate as well as resonant but I pump the same power into both where does the "lost" power go in a non-resonant antenna? I know that in practice, there will probably be more loss in the feedline because of higher SWR. I also know that there will be some loss in the tuner which you probably won't need with a resonant antenna.

The typical doublet with ladder line is definitely a compromise mostly because of the complex radiation pattern on the higher bands. A half wave dipole of sufficient height will have a simple lobe on each side so it will probably send a better signal in the direction you want if you orientate it properly.

None of these losses and compromises are the same as saying "resonant antennas radiate better".

 

A "doublet" fed with ladder line or remotely tuned at its center feed point is NOT a compromise dipole antenna but a performant one as long as a simple rule is followed.

To avoid pattern distortion on higher than fundamental resonance bands, an l/2 dipole at l/2 from ground may be used not higher than its second harmonic.

For example, a 80m halfwave dipole works absolutely fine on 40m but suffers from strong pattern distortion on higher bands.

Similarly, an l/2 14MHz horizontal dipole at l/2 from ground shows no pattern distortion up to 10m, working great on 20-17-15-12-10 even outperforming (on higher frequencies than its fundamental) single band resonant dipoles fan or trapped ones.

(A 14Mhz halfwave doublet shows the gain of a 5/8l radiator at 17m, 3/4l at 15m, 7/8l at 12m and 1l at 10m.)

 

An unrelated question but does anyone here understand the threading or sorting on this forum?

When I open this thread the "breadcrumbs" at the top are this:


But if you browse the "Amateur Radio News" forum it's nowhere to be seen.