any other antenna lengths beside 102 that dont need a coil ?

[BOOTY MONSTER]

so a 102-108 inch 1/4 wave antenna doesnt need a coil to properly tune to 11 meters . why is that ?
and as stated in the title . are there any other antenna lengths beside 102 that dont need a coil to tune properly at 11 meters and not shoot the signal straight up like a full wave would ?

 

[W5LZ]

Not quite as simple a question as you might think. Basically, 'cuz it has the required reactances to produce something close to 50 ohms of impedance when grounded correctly (that 'ground' is the other half of the antenna, making the thingy a 1/2 wave for all practical purposes).
Depending on the configuration, multiples of 1/4 wave length can be made to do the same thing, sort of. Speaking of verticals, a half wave length vertical over a 1/4 wave length 'ground' will get you close to 50 ohms, sort of (not exactly, but close enough). Also depends on how the thingy is shaped to some extent.
Wanna know what all that 'depends' consists of? Find a text on antennas and start reading about impedances, reactances, how they are produced, changed, etc, etc. T'ain't easy, but eventually you get enough sand pounded into that hole to figure it out, I think, maybe?
- 'Doc

(By the way, a full wave antenna does not radiate everything straight up. Bunch of it goes in other directions. )

 

[Beetle]

It seems that you're talking about a typical quarter-wavelength vertical antenna...

Cut yourself a random length of wire. Four feet, fourteen feet or whatever. You have just cut yourself a quarter-wavelength antenna for SOME frequency (and you can calculate what that frequency is). With a proper ground plane or counterpoise, the antenna you just cut will work just fine on and within a small bandwidth on either side of that frequency, without any coils or other add-ons. It's only a quarter-wavelength on that ONE discrete frequency, though.

For 27 megahertz work (CB), a quarter-wavelength happens to be around 103 inches (use the formula: 234 divided by the frequency in megahertz). Assuming a frequency of 27.2 MHz, that figures out to just a tad over 103 inches.

It doesn't require a loading coil or other devices because it's a resonant antenna. At resonance, inductive and capacitive reactances are equal in magnitude, and since they have opposite algebraic signs, they cancel each other, leaving only the resistive component which, with typical ground losses, should be close enough to 50 ohms to provide a good match to the coax.

 

[BOOTY MONSTER]

thanks guys for the informative replies .

 

[W5LZ]

Something to think about, not really important as such, but interesting to know.
Before about 1935 or so most feed lines were of the single wire, or parallel wire type. Single wire for those antennas (usually a vertical) that used ground as the 'other' part of it's self. Seldom were any of them even close to 50 ohms, usually something either quite high or quite low in impedance. A transmitter's output impedance wasn't usually anywhere close to 50 ohms either. People didn't much care what the SWR was, most didn't even know what SWR meant. Then sometime in the '40s, during the war, when trying to put radios into airplanes, radio people decided to try using a coaxial type feed line 'cuz parallel feeders were not easy to install in planes. The impedance of those coaxial feed lines wasn't much worried about to start with, it ranged anywhere from about 50 ohms to over 90 ohms, sort of. As it became evident that there were problems when the transmitter's impedance, the feed line's impedance, and the antenna's input impedance weren't at least 'close' (the cable kept getting 'burned'), things started to get redesigned for a 'common denominator' for all three. That coax getting burned up was a product of the materials available at the time and production methods/tolerances, sort of. Still that way, especially with solid state 'pickiness' and no tunning ability with the radio's output circuit. That's when SWR started to rear it's ugly head. The common feed lines before coax had such tolerances that SWR just didn't matter a lot, along with the fact that there were knobs to adjust the output circuitry to get sort of close sometimes. (Common tuning indicator was a light bulb... brightest light meant best tunning. Didn't much matter what the SWR was since the feed line wasn't all that 'delicate'. The finals either. Weren't no SWR meters anyway.) Determining the length for an antenna has to do with resonance. Has nothing to do with SWR. Some particular lengths sort of came close to 50 ohms, but none were really close at all, and it didn't matter (them knobs on the radio again... or the tuner).
So, when you get down to the 'nut-cracking', impedance mismatches didn't make a huge difference unless you were using a feed line that didn't like those mismatches. There were some losses of course, but they were not a biggy. Some of those 'old-timey' thingys did have a useful purpose, and still do...
- 'Doc


Where did all that @#$% come from? My 'mail-person' asked if I wanted some old 'QST's from the '50s and '60s. I never turn 'nuthin down that's free! Got a couple of stacks three feet deep of magazines. Whole @#$% pot full of interesting stuff in there! No SWR meters available till mid to late '50s was one of those interesting thingys. And some thought explained why. Ain't even half way down that first stack yet! '13' element beams for about $6.75?? Good grief!

 

[CDX-007]

Re: any other antenna lengths beside 102 that dont need a co

so a 102-108 inch 1/4 wave antenna doesnt need a coil to properly tune to 11 meters . why is that ?
and as stated in the title . are there any other antenna lengths beside 102 that dont need a coil to tune properly at 11 meters and not shoot the signal straight up like a full wave would ?

- BTW, a 3/4 wave also loads nicely, but requires a 1/4 wave counterpoise / radial system for close to 50ohms impedance.

The 3/4 wave has both a high radiation angle like the 1/4 wave and a higher radiation angle than the 1/4 wave.

Great 27mHz antenna for those stuck in deep canyons, though at ~27' long it can be a construction challenge.

Here's an interesting website.

http://www.cebik.com/gup/groundup.html

 

[lords]

Re: any other antenna lengths beside 102 that dont need a co

so a 102-108 inch 1/4 wave antenna doesnt need a coil to properly tune to 11 meters . why is that ?
and as stated in the title . are there any other antenna lengths beside 102 that dont need a coil to tune properly at 11 meters and not shoot the signal straight up like a full wave would ?

yeppers a 109" on a superscanner antenna ....

 

[Switch Kit]

I surely missed the boat on this one ? Most Francis fiber glass antennas from 3 ft to 8ft even (96 inches) have always seemed to tune nicely without a spring or coil on them , would this mean they are loaded inside of them ? I remember the 8 footers Wheeler Dealer and Amazer having 2 to 4 (or something like that) thin pieces of copper wire going from bottom to top , but I suppose that would surely make me wonder why a 3 footer or any of the shorter models would match out so easily with springs or coils. IM sure I must be missing something here.

 

[Master Chief]

Re: any other antenna lengths beside 102 that dont need a co

BTW, a 3/4 wave also loads nicely, but requires a 1/4 wave counterpoise / radial system for close to 50ohms impedance.

Are you aware that 5/8 antennas are actually loaded to the next 1/4 wave in order to get to the 50 ohm feedpoint? The next length is a full wave, or every multible of a 1/4 wave. The problem with the 3/4 wave antenna is exactly what you pointed out, the high angle of radiation. A .64 wave length is the longest antenna that maintains the maximum amount of gain. This is why the 5/8 (.625) wave is such a popular design. The broadcast industry learned this back in 1934 (or so).

 

[CDX-007]

Yep, there's just nothing like a good 4-radial, metal .625-.64 wave GP for overall performance.
It's no beam but for a one-antenna-does-all it's definitely my antenna of choice.

I love to see Ops convert that old POS Radio Shack .64 to a real-world Penetrator by rebuilding the feed system. Those dang antennas get about 2mHz @ 2:1 swr!

I'm out of my kits right now but should have a few more coming in soon.

 

[Marconi]

Re: any other antenna lengths beside 102 that dont need a co

BTW, a 3/4 wave also loads nicely, but requires a 1/4 wave counterpoise / radial system for close to 50ohms impedance.

Are you aware that 5/8 antennas are actually loaded to the next 1/4 wave in order to get to the 50 ohm feedpoint? The next length is a full wave, or every multible of a 1/4 wave. The problem with the 3/4 wave antenna is exactly what you pointed out, the high angle of radiation. A .64 wave length is the longest antenna that maintains the maximum amount of gain. This is why the 5/8 (.625) wave is such a popular design. The broadcast industry learned this back in 1934 (or so).

Well guys this study that MC presents as the definitive study to the superiority of the 5/8 and .64 wavelength style antenna has been argued at higher levels of understanding over and over I'm sure.

My source is ARRL Antenna Compendium, Volume I, dated 1985. The article is entitled "The 5/8-Wavelength Antenna Mystique" by Donald K. Reynolds K7DBA, and it addresses the technical paper published in 1924, that I believe MC suggested. I have searched the Internet for the article, "On the Optimum Wavelength for a Vertical Antenna over Perfect Earth" which he mentions to no success. It was an article written in, "Proceedings of the Institute of Radio Engineers, 1924." Since I cannot prove anything factually, I will take what these articles say at face value. The only contention I make about the study in question is noted in the title only.

The title of the study itself explains in part that the performance of the model examined by the author, Stuart Ballantine was conducted over a Perfect Earth Ground. His model was consider as being over perfect earth and the quality of the earth ground computed in the case study was both infinite in extent and conductivity. Both of these factors are likely impossible to attain in a real world setup, so we already have a clue as to a somewhat skewed result, excepting maybe in Theory. As close as we can probably get to a perfect earth ground is over salt water and I don’t believe that is really even close to a perfect ground of infinite extent and conductivity.

We cannot ignore these significant conditions placed on this study. These two distinctions are just too significant when considering antenna performance in a real world setting as to be ignored by setting them perfect. I can’t argue that it was wrong to conduct the computations as noted, I just say that when we relate these results to real world experiences we must consider the affects of the earth on the antenna, and this test basically ignored those affects. Same argument as is often made about isotropic results.

It may be true that in the theory developed from their study there is a 3 db advantage using the 5/8 wavelength radiator. However, Cebik raised the same question about the affects of earth in several pieces he did on “The 5/8-Wavelength Mystique.” He reported in his work that the better advantage is more than likely in the range of less that 1 db advantage for the 5/8 wavelength over the 1/4 wave all things considered.

I don’t think we can ignore words like “over Perfect Earth” when we read about this study and then use its results for comparisons.
---------------

 

[Marconi]

Yep, there's just nothing like a good 4-radial, metal .625-.64 wave GP for overall performance.
It's no beam but for a one-antenna-does-all it's definitely my antenna of choice.

I love to see Ops convert that old POS Radio Shack .64 to a real-world Penetrator by rebuilding the feed system. Those dang antennas get about 2mHz @ 2:1 swr!

I'm out of my kits right now but should have a few more coming in soon.

007, I think you refer to your kit as improving bandwidth for the RS .64. What does theory tell us happens when we are able to expand bandwidth an appreciable amount?

I don't suggest that increased bandwidth isn't important in some situations, but isn't improvement in bandwidth and match always to some extent a compromise against gain.

 

[Switch Kit]

And I thought I could get deep :? You guys get so deep with this stuff that I would pesonally think you all should make loads and loads of money for your time and know how. I can only hope it certainly makes your radio experiances all the better. If I couldn't have kept it simple over the years concerning any antenna I ever owned , I would have been gone a long long time ago. Enjoy

 

[Marconi]

Yep you may be right Switch Kit, I should have directed my remarks to MC since he raised the issue about the 5/8 wave antenna being the premier industry standard for the broadcast radio industry.

Maybe MC or someone else can provide some evidence that the broadcast industry actually does use the 5/8 wavelength idea as the preferred standard as opposed to other wavelengths like the 1/4 wave or 1/2 wave element.

I feel I responded with a different point of view that was pretty simple since we all probably realize that the Earth's surface and location has many significant and different affects on the performance of antennas. I went to the trouble of suggesting why and I also provide a couple of references to support my thoughts on the subject and I tried to keep it simple. MC likely will not agree with me, but he may understand what this point of view is about.

SK, would we be better informed if I simply claimed to everyone that the broadcast radio industry has not benefited from the use of the 5/8 wavelength antenna since 1924 and that 1/4 wave and maybe even 1/2 wave elements still prevail?

Or, maybe I should have just posted some smiley faces and said MC is full of it. Maybe that would have better informed us all.

 

[Switch Kit]

it's all good with me Marconi , I didn't mean to offend anybody , even after 20 some years around this hobby , this stuff tends to get so deep for me at times I just have to personally back up from the subjects on hand , not that IM hopefully learning something along the way , it just tends to get so deep into a subject that it's basically out of my league if you know what mean ? IM very impressed most of the time , but then again , IM not sure if I even understand certain subjects to begin with to be impressed with in the first place.(You confused yet ) That's kind of what I meant by saying , Many times in this hobby, and life for that matter , I need to keep it as simple as I possibley can, If I don't , I have a way of screwing things up for myself. It's a blessing to know a few things and it's even more of a blessing if I can learn something , but there's just to much to know in the first place , so underneath it all ,I only know a little and that's enough for me for the time being. Peace