Ham Radio Forum

Dear Guys and Gals, hi!

This is my very first posting on this fine Forum and I'd like to say Thanks to the fellow hams that keep this site always in ship shape form.

But I'd also ask you a question:
What is the best way to feed the 8JK antenna? I remember reading an article about this antenna in an old issue of 73 Magazine (QST, perhaps), probably in the 70's (the article showed how to build the antenna and the feed line), but I can not find the magazine anymore, nor the article. All I recall is that the feeding was made of an (open? closed?) stub, and one had to look for the correct (low impedance) feed point by sliding a shortening rod along the stub. Does it ring any bells? The article went to the extent of teaching how to build the open-line, using wooden dowels immersed in boiling paraffin or, alternativelly, plastic dowels). That's all I remember.
Could anybody point me in a right direction? I'd be gratefull for any comments.

Thanks a lot!

--
PY1VHF * Anderson Colla
GG97co * py1vhf (( at )) amsat.org
Maca

I don't recall the article, but I can suggest some good impedance matching and feed-line alternatives.

The 8JK is a great antenna. However, many design variations have been experimented with and recommended in various articles since W8JK first designed the antenna and published an article in about it in 1930's, so more specific information is needed about the design you are using before recommending a feed-line and impedance-matching system. Most hams feed 8JK's with balanced open-wire feed-line and then use an antenna tuner on the radio-end for impedance and unbalance-to-balanced line matching. That generally works well, but it is possible to impedance-match and balance other ways and so other types of feed-lines, such as coax, can be used. The practicality of those alternate methods depends on the factors below.

1) What are the end-to-end element lengths?

2) What is the spacing between the two dipoles?

3) What is the antenna height above ground?

4) Will you be using the antenna on a single band or on multiple bands and which band or bands will it be used on?

Hello, Skywave, thanks for the prompt response!

I intend to build a 40 meters band W8JK, which can be used on several harmonic related bands, with gain, but it will be used primarily on 40 meters.
I understand this is an bi-directional antenna, and I wil not build any rotator or any rotating mechanism. It will be fixed, in a NE/SW position (beaming Europe / South Pacific. I live in Rio de Janeiro).
In an article in QST (June 1982), Dr. Kraus himself showed a new form of feeding the antenna, using a trombone match. I thought it pretty awkward. I prefer to use some sort of stub and try to find a low (50~80 ohms) impedance point, and then feed it with RG-213 to the rig. But I do not know what the stub lenght would be, nor its impedance nor whether it would be closed or open.
In case I do not find any further info on the stub subject, I will be feeding the beast with ladder line, straight to the tuner.

I'm still in the planning, and the following dimensions will be used:

Center Frequency: 7100 kHz
Element lenghts: 20.0 meters (65.62 ft)
Spacing (1/8 lambda): 5.0 meters (16.40 ft)
Height: abt 17 meters (abt 55 ft)

Any further input is appreciated, Skywave!

If the antenna is made from 14 AWG bare-copper wire, if the insulators have near-perfect characteristics, and those dimensions are used the resonant frequency will be 6.424 MHz.

At 6.424 MHz:
The feed impedance will be 4.545+j0.0 ohms.
The forward gain will be 8.7 dBi
The front-to-back ratio will be 10.62 dBi
Maximum radiation will occur at a vertical angle of 31.3 degrees.

At 7.1 MHz:
The feed impedance will be 6.538+j82.768
The forward gain will be 8.64 dBi
The front-to-back ratio will be 4.23 dBi
Maximum radiation will occur at a vertical angle of 28.6 degrees.

Both those feed impedances will result in large power losses and consequently low antenna efficiency using any practical matching method. The antenna also will have a very narrow frequency bandwidth. That is especially true at 7.1 MHz where the feed reactance will be 12.66 times the feed resistance. I suggest the following two changes to your design to greatly reduce losses and change the antenna performance from very poor to very good:

1) Change the element lengths to bring the antenna to resonance at 7.1 MHz.

2) Change the two driven elements to folded dipoles, or even better if you are willing, to dipoles folded three or more times.

If those changes will be acceptable I will calculate resonant dimensions and new antenna characteristics, and will show how the new design can be matched to coax efficiently. However, before I make those calculations let me know the gauge or diameter of the actual wire you will use, whether it will be copper or aluminum, whether it will be insulated, if so, what the insulation material and thickness will be.

Hi there, skywave, good morning.

Many thanks for the excellent input. You're helping me a lot.

Down to business:
Although all the valid considerations you've made, concerning the elements configuration, I'd prefer to stick to the original Kraus' project, using a pair of half wave dipoles, instead of the suggested folded dipoles.
Regarding the element lenghts, it would be ok to adjust them, so they will resonate at 7100 kHz. What lenghts do you suggest?
Is the half-wave formula aplicable here? I'd be using #14 AWG solid copper wire, insulated (I do not know what the insulating material is, probably some sort of non-inflamable plastic) and ceramic insulators.
The distance between elements could be anything from 5 to 7 meters (16.4 to 23 ft).
As it will be used primarily for DX, low (40 degrees maximum)take-off angles would be a must.
Thanks a lot, old man (I'm assuming that you are a man. It is impossible to tell from the nickname!). The information you gave me so far is a lot better than that I managed to get from surfing the web.

And please forgive my (several...) English mistakes. My mother language is Portuguese.

anderson colla
py1vhf

Your English is good and much better than my Portuguese. I used to go to Rio de Janeiro on business occasionally and I dated a Brazilian woman there a few times. The relationship was going well until I bought her some flowers and attached a note I had tried to write in Portuguese using an English-to-Portuguese dictionary. She became extremely angry when she read the note and refused to see me again. I didn't keep a copy and never learned what the problem was, but I obviously got something seriously wrong.

As to the antenna, the two 8JK radiators must be cut slightly shorter than for a single half-wave dipole, because of the effects of tight mutual coupling between them and cross-feed inductive reactance. With your wire-size, wire insulation, and height above ground their overall lengths must be reduced to approximately 17.81 meters to increase the resonant frequency to 7.1 MHz. With that length change, perfect end-insulators, and no other conductors nearby:

The feed impedance will be 5.39+j0 ohms.
The bandwidth will be 33.4 KHz for a VSWR less than 1.5 to 1.
The bandwidth will be 67.4 KHz for a VSWR less than 2 to 1.
The forward gain will be 9.0 dBi.
The front-to-back ratio will be 10.9 dBi.
Maximum radiation will occur at a vertical angle of 29.4 degrees.

The useful transmitting bandwidth will be significantly less than for a single half-wave dipole at the same height, because the tight mutual coupling between radiators raises the antenna's Q. Because the bandwidth will be narrow, and because the resonant frequency probably will be changed by nearby objects, you should plan on initially making the elements slightly long and then trimming them to resonance after installation. The bandwidth would be significantly wider with folded dipole radiating elements, which would reduce the seriousness of detuning.

The 5.39 ohm feed impedance will cause a significant amount of power loss as heat, because the feed current will be very high resulting in an even higher RI^2 power loss. An RF transformer wound on a toroid core (a balun) will be the simplest way to match the antenna to 52-ohm coax cable. A 3:1 primary-to-secondary transformer turns-ratio provides a 9:1 impedance transformation. 9 x 5.39 = 48.51 ohms, which will be a close match to 52-ohm feed-line.

A transformer with that primary-to-secondary turns-ratio will be easy to make, but it will be important to use an unusually large-gauge secondary wire-size and a toroid core that has an unusually large cross-sectional area to minimize transformer losses where you will be transforming to such a low load-impedance.

If folded dipoles were substituted, the antenna feed impedance would become 21.56 ohms, the antenna feed-current would drop by a factor of four, and the power loss would drop by a factor of 16. Even better, if the dipoles were folded one more time, so each radiator consisted of three parallel wires with a feed-point in the center of one of them, the antenna feed impedance would become 48.51 ohms, there would be no need for impedance transformation, and the useful bandwidth would be significantly greater. However, it would be necessary to slightly reduce the radiator lengths to maintain resonance at 7.1 MHz and a 1:1 balun (very low loss) would be needed to match unbalanced coax cable to the balanced antenna.

skywave wrote:Your English is good and much better than my Portuguese. I used to go to Rio de Janeiro on business occasionally and I dated a Brazilian woman there a few times. The relationship was going well until I bought her some flowers and attached a note I had tried to write in Portuguese using an English-to-Portuguese dictionary. She became extremely angry when she read the note and refused to see me again. I didn't keep a copy and never learned what the problem was, but I obviously got something seriously wrong.

It's a pitty you did not keep that note! I too wonder what you said that made her so upset! One must be careful with these translations...

Anyway, thanks again for helping me out. Have a nice Sunday!
On the antenna, ok, you are almost convincing me to substitute folded dipoles for folded dipoles (tripoles would be too cumbersome for me to build and erect). I never thought the impedance would be so low and the power losses so high!
You said the "The front-to-back ratio will be 10.9 dBi". Did you mean "front-to-side ratio"? Or I'm confused?

Sometimes I think in building the 8JK according to the original specifications (regarding antenna feeding), and feed it directly with open-wire. It would be much less complicated. At the same time, it would not be so fun!
In case I decide and use open-wire straight from the feed point to the tuner, should I worry about the feed line lenght?

Thanks for the data you sent already. It is being very useful, believe me!

I just realized that I probably misunderstood the element length value you specified. The antenna characteristics I posted above are based on "overall lengths" of 17.81 meters with overall meaning from the one end of the antenna to the other.

The radiation patterns will be bidirectional and the antenna feed impedance will be much higher with elements approximately 20 meters long on each side (40 meters overall). I will recalculate will elements approximately twice as long.

Ok, thanks for your time and patience.
anderson
py1vhf

The 8JK has the characteristics below with 20-meter long 14 AWG insulated copper elements on each side of the two center feed-points (40-meter radiator lengths overall), balanced open-wire cross-connect feel-lines made from 14 AWG insulated copper wires spaced 2-inches apart, and an antenna height above ground of 17 meters.

Frequency 7.1 kHz
Feed impedance at center of cross-connect feed-lines 27.7-j632 ohms
Power gain in each main lobe 8.7 dBi
Maximum vertical radiation angle 27.5 degrees

The 27.7-j632 ohm feed-impedance would be easy to match to coax cable at 7.1 kHz. If that was the only frequency you cared about, you could simply insert 7.16 uH coils in series with each feed connection to tune out the reactance (however, it would be important to orient the two coils at 90 degrees with respect to each other to avoid mutual coupling between them). If the coils each had Q's of 200, they would raise the feed resistance to 30 ohms and a 2:1 balun could be used to provide a 1.16:1 VSWR match to 52 ohm coax. Alternatively, either an open or closed-end balanced open-wire transmission line stub could be cut to tune-out the 632 ohms of feed-point reactance at 7.1 kHz, instead of using coils.

However, either of those matching methods will result in a very-narrow antenna frequency bandwidth. The VSWR will be less than 1.5:1 over only 22.8 kHz. It will be less than 2:1 over only 56.5 kHz. The VSWR will be 6:1 at 7.0 MHz and at 7.2 MHz. That is why most people use open-wire feed-line down to an adjustable antenna tuner inside the shack with 8JK antenna elements that length. Wideband impedance matching without adjustment is theoretically impossible, regardless of the coupling method used.