Grounding a Multiband Vertical Antenna for Better Swr Readings

For the past couple of weeks, I've been playing with end-fed wire antennas. Earlier I go into the nitty-gritty details, let me outset make a distinction between end-fed half-moving ridge antennas, such equally the ones sold by LNR Precision and end-fed wires that use some kind of tuning to achieve a l Ω output impedance.

End-fed, half-wave antennas (EFHWs) are a half-wavelength long and are resonant antennas on the band of interest. They employ some kind of matching network to trasnsform the very high impedance at the end of a half-wave wire to about 50 Ω. By and large, they are non usable on bands for which they are not a half wavelength long. Yous can't, for example, more often than not use a 40m EFHW antenna on 20m.

Terminate-fed wire antennas are a different animate being. They are non a half-wavelength long, meaning that, if you cull the length of the radiator wisely, the impedance at the cease of the wire will not exist as high equally the impedance of a one-half wavelength long wire.

In many cases, the impedance can be transformed with the help of a ix:1 unun (unbalanced input to unbalanced output). See the figure at right. A 9:ane unun is a transformer that reduces the impedance at the input by a factor of 9. Then, if you lot connect a length of wire that presents an impedance of about 450 Ω to the input, yous'll get an impedance of virtually 50 Ω on the output.

In fact, the 9:1 unun that I built is actually an autotransformer. Here's a video that talks a little chip about autotransformers.

It'due south relatively easy to build a 9:1 balun. I of the most common designs is to wind nine turns of a trifilar winding around a toroid core. Trifilar means that in that location are three wires wound simultaneously around the cadre. I'g non sure why in that location are nine turns, instead of say eight or ten, but I suspect that it's a compromise between size and coupling. Ix turns yields sufficient coupling to ensure that the impedance transformation will take identify without taking upward too much space.

By the way, the ratio 9:1 isn't really magic. You lot could cull to build a transformer with 7:1 or 12:ane ratio, but it just so happens that it's much easier to build a 9:1 transformer than a 12:i transformer.

I built i on a T80-2 powdered iron core, using some 22-ga. wire that I scavenged from some four-conductor cablevision (see photograph at left.) I got a little fleck lucky in that the T80 core has a diameter just big enough to accommodate nine turns. Using dissimilar colored wires (ruby-red, blackness and white) fabricated information technology easier to wire information technology up properly.

I didn't do much engineering when it came to selecting the parts. I just happened to have a trivial bag of T80-2 cores that I'd purchased cheap at Dayton a couple of years ago. The short length of four-conductor cable was something that I'd salvaged from some previous projection and had simply thrown into my "wire box."  I haven't washed the calculations, but equally built, I'd guess that it's good up to 25 W or and so. If you're shooting for an unun to handle more ability, then go with a T130 core and heavier gauge wire.

At that place's also some question nigh which blazon of core to apply. Some people wind their unun on ferrite cores instead of powered iron cores. One manufacturer even goes so far to say that they utilise a "custom mix" instead of one of the standard ferrite mixes (although I find information technology difficult to believe at the relatively depression quantities that they must be purchasing that they're getting a truly custom mix). My friend, Thom, W8TAM, built his 9:1 unun using anFT82-61 cadre, and it works great. G3TXQ has performed a number of experiments with different core types, and with the antenna he used, found Type ii powdered iron cores to be preferable.

So, how long a wire should you utilize for the antenna? It really depends on what bands you desire to work. Mike, AB3AP, has calculated the lengths that give skillful results on various bands. Jack, VE3EED (SK), has also made this calculation. They differ slightly considering VE3EED used the center of the bands in his calculations, while AB3AP used the center of the CW portion of the bands.

Last Saturday, I played around with an end-fed with a 36-ft. radiator and counterpoises of 13-ft. and 25-ft. To be honest, I wasn't actually happy with whatsoever of the configurations. The best I was able to exercise was achieve an SWR of 2:1 on 40m with the 36-ft. radiator and the 13-ft. counterpoise. Neither configuration yielded a satisfactory friction match on 20m.

Thom, on the other hand, used his 9:1 unun with a 30-ft. radiator and got fantastic results. He got groovy signal reports from an NPOTA station, a special event station in Georgia, and an operator working aeronautical mobile over Nebraska. So, in that location's more experimentation in my hereafter.

Update 8/19/2016

Every bit I mentioned before, I wasn't very happy with the results I was getting with the 9:1 unun that I had congenital earlier. And so, yesterday evening, I went over to W8TAM's house to compare his ununs to the 1 I merely built. Equally I noted above, Thom had great success with his a couple of weeks agone.

The first affair we did was bank check that I had wired it properly. The only thing that nosotros found is that instead of 9 turns, I had only wound viii turns on the T80-2 core. I didn't recall it would make that large a difference, only Thom had a lot of wire, so we rewound the unun, this time making sure that I wound nine turns.

We connected ii 1 kΩ resistors in parallel  (to give united states an input impedance of 500Ω) from input to ground and measured the output impedance on Thom's Rig Expert AA-170 antenna analyzer. As I suspected, the extra plough made little difference. The SWR was half-dozen.5:1 on 7150 kHz.

(Every bit an aside here, I have to comment on the AA-170 antenna analyzer. In a word, it'due south fantastic. One of the functions we used, for case, measures the SWR of antenna at the midpoints of all the amateur bands. This role is just perfect for testing the frequency response of baluns and ununs. It'southward besides graphs SWR across a frequency range. And, on top of all that, it'due south about half the weight of my Palstar antenna analyzer. I think I'grand going to dump the Palstar and get a RigExpert.)

At that point, we figured that the only thing it could be was the cadre. Fortunately, Thom had an FT82-61 ferrite cadre that he'd used to wind a QRP 9:i unun. We cut three more lengths of wire, wound the unun, connected the 500 Ω load and measured the SWR.

WOW! The thing worked exactly as predicted! Here are the measurements:

	T80-2	FT82-61
7150	half dozen.v	ane.2
10125	4.1	1.1
14175	two.9	1.0
18118	2.3	ane.1
21225	2.0	one.2

This was very puzzling and aggravating. As I noted in a higher place, G3TXQ institute that the #ii powdered iron cores gave the best results. Later on a little Googling, I also found another ham,VK6SYF, who had success with a #two core (http://vk6ysf.com/unun_9-1.htm). My friend KA8BMA congenital one using a T106-two core, and it seems to exist working right.

I bought the cores from a reputable dealer, so I don't remember that they're bad, but I don't know  how else to account for the difference in performance. If you have any ideas on that, I'd dear to hear them.

After getting that out of the way, we got into a discusssion of whether or non I really needed a 9:one unun at all. The two radios that I use for portable operations are the Elecraft KX1 and the Elecraft KX3, both outfitted with antenna tuners. Both of them seem able to melody just most any length of wire non an verbal one-half-wavelength, and that being the instance, why bother with the nine:1 unun? The unun would just introduce more loss into the organization.

Sounds like more experimentation is in order.

UPDATE 8/22/xvi

A couple of days ago, I e-mailed G3TXQ about my lack of success building a 9:one unun with a T80-ii core, even though he seemed to have the all-time success with a #ii core. He said, "Using #2 cores will not give yous a very accurate nine:1 impedance transformation, particularly if you use something as small as T80 size." I found this kind of puzzling every bit he had said that in his experiments, he got the best results with a #two core.

Anyhow, since my friend Rich, KA8BMA, had wound his 9:1 unun on a T106-2 cadre, I idea I'd try this, too, and I but happened to have one in my box o' toroids Well, wouldn't y'all know it, the measurements are much, much better with the T106-2 core than they were with the T80-ii cadre.The SWR measured ane:i on 20m and 17m, something like 1.eight:one on 40m, and i.5:i on 15m.

The frequency response of this unun isn't as apartment as the unun wound on the ferrite cadre, but it is certainly much improve than the unun wound on the T80 core. The T106 core is only a quarter inch larger in diameter than the T80 core (1.06-in. vs. 0.eighty-in.). I wouldn't have thought that a quarter-inch in diameter would make that much difference, but it did.

In researching this further, I discovered why. Co-ordinate to VK2TIP's explanation of wide-band RF transformers, the impedance of the chief winding should exist at least fives times the input impedance. Since my test input impedance is 500 Ω, that means the impedance of my principal winding should be at least 2500 Ω.

Using the online figurer at toroids.info, at a frequency of 14 MHz, and a primary winding of 27 turns, I become the post-obit values:

	AL	Ω
T80-ii	v.five	352.7
T106-2	thirteen.5	865.7
FT82-61	79	5066

The offset thing to notice is that the inductance gene, A_L, for the T-106-2 core is most twice that for the T80-2 core. I erroneously thought that they would be the same since the core material was the same. The college value yields a higher inductance, and therefore, a ameliorate transformer.

Even so, the inductance is far from five times the input impedance. That'south why the FT82-61 ferrite core works much meliorate for this application. With an A_Lof 79, the primary has an impedance of more than than 5000 Ω, which gives a very skillful transformation.

As Steve, G3TXQ, pointed out in his due east-mails to me, it actually doesn't matter if the impedance transformation is exactly 9:i or some other value—especially if you volition too be using an antenna tuner. In that case, winding your unun on a T106-2 cadre—or even better a T157-2 core that has an A_Fifty of xiv—volition work OK. If you lot're like me, though, and want a ix:i unun to really give you lot a ix:1 transformation over near of the HF bands, then employ a ferrite core.

One final notation: A very nice feature of the toroids.info calculator is that it not only calculates the impedance for a particular number of windings, but also the length of the wire that y'all'll need. On my beginning attempt at winding an unun on a T106-two core, I profoundly underestimated how much wire I needed and ended up throwing away that wire. I could have avoided doing that if I'd used the calculator.

carpenterthisidiever1981.blogspot.com

Source: https://www.kb6nu.com/playing-end-fed-wire-antennas-91-ununs/

Share this post