Home Brew Antenna Question

[RossJ]

I just constructed a horizontal 5/8 wave, 11 meter antenna using two 3ft Fire Stick firefly (5/8 wave) antennas set 180˚out from one another, mounted on a 'C' bracket… one connected to the low side and one to the high side.

It looks kind of like this: ------n------ .

When I check it with my SWR meter, channel 19 is almost perfect (about 1.05), but as I go toward either end of the radio range (channel 1 or 40) the SWR goes up to about 2.5. Does anyone have any idea why the center-range channel would have a low SWR, but increase as the frequency is set up or down from there?

Thanks

[Deleted User 14541]

Sounds like you have built a narrow banded antenna. Maybe longer antennas would help.

['Doc]

It sounds perfectly normal for any antenna.
And just for grins, those two '5/8 wave' loaded antennas were never a 5/8 wave to start with, and still aren't when assembled as you've described. That length expressed in wave lengths or fractions of wave lengths means exactly what it says. A 1/4 of a full wave length, a 1/2 of a full wave length, or 5/8 of a full wave length at the frequency of use. So what's a full wave length at 27.200 Mhz?? Do your 'size' determinations with that number. What you have is much closer to a center fed 1/4 wave length loaded antenna than anything else. And since the physical length, not the 'electrical' length is what determines the shape of an antenna's radiation pattern, I don't think I'd count on a very good one. It's still fun to play with though, ain't it?
- 'Doc

[RossJ]

Hey, thanks for the input. The URl below shows what I was working toward, but trying to do with about two feet less antenna: [Please login or register to view this link]

I'm still going to mess around with it a little. The receive characteristics seem pretty good and I can transmit safely (under a 1.5 SWR) from channels 12 to 26. Like any antenna, the higher I can get it and the further from any buildings, the better it behaves. Regrettably, it falls short of the one in the 'Stealthy Homebrew' example.

Ross

['Doc]

Actually, you will probably be 'safe' up to about a 2:1 SWR, which should get you the majority of the 40 channels. That 2:1 isn't as 'nice' as a 1.5:1, but it's still usable.
SWR just doesn't tell you much about how well an antenna is working, it only tells you what the impedance match is like, which is all it was ever intended to do. A very good example is a dummy load. It has a very nice SWR, but that certainly doesn't mean a dummy load is a good antenna.
- 'Doc

[RossJ]

I get what you're saying, Doc.

It's been a fun little science project. I have a couple of mod's to encorporate before moving on. I guess I expected a little better/broader impedance match than I got. I'd hoped to get a functional compact dipole, but I think it's more of a fancy bird perch.

[PoeDunk]

Pretty much any time you add loading coils into a dipole you will make the overall bandwidth narrower. Those wire wound fiberglass sticks are just loading coils in different configurations. They work well for truckers because they tend to use the channel 19 frequency and a few close to it. But even then you would have to evaluate what exactly 'works well' would mean. to the truck driver that is on the move all the time they actual performance of the antenna may be hard to determine, so they just figure that it works well and their mobile amp doesn't fry it.

When you actually try to use such antennas for fixed station operation, the shortfalls are more evident. If you are really tight for space, you may want to look into a concept called 'linear loading' an antenna. It is used by hams that have space limitations because it seems to be a design that allows for the best bandwidth in the shortest space. All shortened antennas are compromises, but some are still better than others.

Let me qualify that last statement about compromises. There are some newer technologies that are actually being evaluated by the FCC as replacements for the older AM and FM broadcast antennas currently in use. Try a google search on EH Antennas. I don't think they sell them anywhere yet, but they give tons of free details about how to make them yourself complete with calculators for building them to your target frequency. I believe the bandwidth may still be an issue, but the overall size is amazingly small. If your in the mood to experiment, why not?!?

There is another type of dipole that you may be able to use. Try google search for "G5RV antenna". It is another antenna that seems to fly in the face of traditional antenna design, but it is easy to build and a lot of people like it. It is a short dipole design that uses an alternative feed configuration.

PoeDunk

[RossJ]

PoeDunk,

Thanks for the informative response. Adequate space is an issue I struggle with. I’m sure my challenges could be overcome with sufficient investment, but (besides the money) there are down sides to putting up and maintaining a 30ft+ antenna tower at my home.
Our attic is in sections with the largest space about 7ft at the peak and about 20ft long. I’m interested in local communications, so I know a horizontal dipole isn’t my best choice. I was looking for a way to maximize in the 7ft vertical space available and the 6ft double FireStik arrangement seemed worth the $30-$40 investment on the chance it would work.
All-in-all, it might be my best option to begin scouting around for salvage metal that I can weld into a tower structure and figure out how to incorporate it into the contours of our house such that the neighbors won’t freak out about a looming pole in their sky view.
At any rate, I appreciate your valued input.

[Ronin]

Could you mount a dipole antenna as an inverted 'V' with the feedpoint at the peak and the legs angled down along the roof joists? If so, look at home brewing (or buying) a double bazooka antenna. When I first moved into this rental house I put up a wire dipole in an inverted 'V' configuration. A month or so later, a friend left me borrow his CB double bazooka and there is definitely an improvement with the double bazooka. The double bazooka was quieter, and transmit and receive was about 1/2 to 1 S-unit better to two people I talked to about 8 to 9 miles away. When a dipole or double bazooka in mounted as an inverted V, it has more vertical characteristics and they work very well. Just an idea since you have a 7 foot peak.

[Circuit Breaker]

I just constructed a horizontal 5/8 wave, 11 meter antenna using two 3ft Fire Stick firefly (5/8 wave) antennas set 180˚out from one another, mounted on a 'C' bracket… one connected to the low side and one to the high side.

In my opinion, FireStik is guilty of false advertising. NONE of their antennas are 5/8 wavelength. They may start with a 5/8 wavelength of wire, but when you wrap it around a length of fiberglass tubing, it becomes a loaded antenna and its physical length will determine its performance. Their 7 ft antenna will outperform their 4 ft antenna even though the length of wire is the same. There was someone, and it may have been on this site, that fashioned a base antenna using a 7 ft FireStik and actually believed he had a 5/8 wavelength antenna and, because of that, it would outperform other antennas such as an Antron 99 or Imax 2000. No one could convince him that all he really had was a 1/4 wave antenna.

[RossJ]

Ronin,

The inverted ‘V’ sounds like a potential solution. Is there a recommended included angle for the ‘V’. I would imagine the tighter the angle, the more vertical characteristics the antenna would demonstrate, but as with most things, there’s usually a range that works best.

I think I’m guilty of buying into the FireStik assertion that their loaded antennas would behave like true 5/8 wave setups. I should have realized that there’s no cheating the laws of physics… if you could really reach out with a six-foot rig, why would anyone erect a twenty-four-foot IMAX?

Now I just need to figure out some alternate use for two three-foot fiberglass rods. I’m open to any suggestions as long as they don’t involve my personal anatomy.

[PoeDunk]

RossJ,

When I first started with a CB base station in a HOA restricted area, I had a simple 1/4 wave ground plane set up in the attic. It was only good for local communications because the ground plane radials were exactly 90degrees perpendicular to the vertical driven radiator. I did this with one of the actual FIrestik branded fiberglass antennas. I mounted it straight up in the floor of the attic and ran 4 ground radials from the feed point along the attic floor like a big X under the Firestik. I situated the radials so that one of them would terminate against a copper water pipe so I could have some semblance of a real earth ground. I later switched the Firestik to a center loaded wilson truck antenna. The kind with a coil half way up the shaft. This improved my local signal and had better overall receive sensitivity.

The reason this configuration does not work for long distance work is the sharp angle of the radials. To get a 1/4 wave vertical antenna to work some longer distances you need to be able to lower the angle of reflected radiation. Commercial 1/4 antennas do this by drooping the radials downward at about a 45deg angle.

The problem you have with the twin firestik configuration is the lack of a good ground plane for reflecting your signal out of the house. In fact, your configuration probably would have worked better if it were horizontal. But the real way to talk to locals is the way I described above. Plus you can toss one of the firestiks.

PoeDunk

[209 first class]

changing the angle of the drooping radials changes the radiation resistance , not the angle of radiation. this is a common misconception, and can be studied in antenna textbooks. helical (spiral wound) antennas have a very narrow bandwidth, your not doing anything wrong. as someone said earlier, they are not really 5/8 wavelength radiators, only the wire is. if you take 8'6" (1/4 wave) of copper wire and helically wind it around fiberglass, ( i have) it will not read a low swr at channel 20. the wire needs to be longer because of inductance /coupling/loading between the windings themselves. 2zero9

[Ronin]

Just tack the feedpoint at the peak and run the legs down at the angle of the roof line. That should be fine. You can experiment, just don't try to run the legs at too hard of an angle or they will start to interact with each other.

[RossJ]

Well, this gives me a lot to chew on. I can envision at least two more science projects in my immediate future before settling on a solution. It’s good to know there are some real options and to gain at least a little insight into what doesn’t perform well and why.

I’ve been working with a 20ft length of RG-58 and doing my SWR readings pretty close to the antenna. I need to run a 50ft length down along one of the house soil vent pipes and do any further projects with a setup that’s closer to my final arrangement. I’ve been using a Cobra 29XLR and an external SWR meter to evaluate the antenna. The meter in the Cobra doesn’t seem to read very consistently. I have a couple Uniden’s, but the SWR readings are pretty much the same regardless of the radio.

I appreciate everyone’s input so far and hope to explore one or more of the described options this weekend.

Thanks.

['Doc]

PoeDunk,
I think you will find that 'EH' antenna is a very good substitute for what is normally spread on gardens to promote growth. I'm afraid that's the nicest way I know of saying anything about that particular antenna. It has been around a while, debated till it's sickening, and it's still horse hockey. You can't change physics because you don't like it.
Radial 'droop'? Whatever works the best, literally. Everything around an antenna can and does affect it's SWR (input impedance) If you can 'use something like a radial drooping to help, why not?
- 'Doc

[rattlesnake]

Maybe the the swr is stable on just one channel or a few maybe the wave length..

[PoeDunk]

I think you will find that 'EH' antenna is a very good substitute for what is normally spread on gardens to promote growth.
- 'Doc

Well Doc, I guess everyone is entitled to an 'opinion', but I've found that actually doing my own research is the best way to go before I really say anything. I have built a 20m EH Antenna and I carry it with me on QRP field day events. Folks can't believe how well it preforms for its tiny size. I'm guessing that you are one of those old-timers that rely on the standard antenna modeling equations. If you really look into the physics of the EH stuff you will see why the old equations are flawed.

I suggest you actually build one and try it for yourself before bashing it. The information is all out there. Just put your prejudice aside and try it.

PoeDunk

[Lost Ram]

I have been researching the EH antenna since I clicked the link here a day ago. Its intriguing to say the least. It kind of looks like the internals of an Imax 2000 but jumbo sized. Its hard to imagine this having any gain. I thought about building one this weekend or next maybe. It looks simple enough anyway.

['Doc]

I think if you trace that 'EH' thingy back a ways you will find that every instance of a commercial build has failed, or not lived up to it's billing. That includes two station that I know of, one in the UK and one in Egypt. I think you will find that it's not a 'prejudiced opinion' of mine, or that I subscribe to any outmoded antenna 'theories'. Trying to give 'Momma Nature's thong a yank like that usually means you get bit...
- 'Doc

[Lost Ram]

Its like a big adjustable capacitor, I wonder if I can drive anything with it.