What W8JI says about a 1/4 wave vs. 5/8 wave

[eagle1911]

Hmm.. If that's the case, then I guess I'll have to live with the losses. Although I do know several people near me who have strung 10-20m dipoles and G5RVs vertically from the tops of trees with little effect on their usefulness, so I'm hoping for a similar experience. I'm definitely missing my old house which was on a 3 acre plot, but I sold it 5 years ago when I married my wife. This IMAX is from those days.. I had it on a 50 ft telescoping mast with no ground plane and I used to have no problem talking wherever I wanted to on 25W PEP.. I miss those days .

 

[HomerBB]

I disagree that he is repudiating the antenna design, I think he tells us the antenna design does present problems, and that an optimum installation or no, can mean the difference in successful use or not.

Here's the problem in our discussion on this point. This sentence:

It seems to me that W8JI is making a case for the repudiation of the idea that antennas, particularly 5/8 verticals, which the Imax2k purports to be, do need a decoupling system in order to maintain stability of operation.

was meant to be:

It seems to me that W8JI is making a case for the repudiation of the idea that antennas, particularly 5/8 verticals, which the Imax2k purports to be, do NOT need a decoupling system in order to maintain stability of operation.

it is not a repudiation of the design, but of an idea with respect to utilizing the design w/o a decoupling system, which I believe he is advocating for vis-a-vis GP radials.

 

[Moleculo]

There are perhaps some simpler ways to state the point W8JI is making. Most people look at a model of well designed 5/8 wave GP antenna, modeled either in free space of over perfect ground and see a radiation pattern they like (for whatever reason) and see that it has some gain over other designs. One of the designs they often compare it to is a 1/4 wave GP because that is also a common antenna. They then leap forward to the conclusion that the 5/8 wave antenna is always better than the others, which is not the case.

it is the ABSOLUTE gain at desired angles that matters

This is the part that people miss. Where do you want the RF to go and does the antenna system that you are going to use accomplish this? Here's a real life example that Happy_Hamer shared with me a couple of years go. He was trying to hit a 2 meter repeater on top of the Sears Tower from his car while in downtown Chicago that had a 5/8 wave antenna on it. He couldn't reliably hold the repeater. He switched it over to a 1/4 antenna and had success every time. Why? because the 1/4 antenna put the RF in the direction that he needed to talk. The point is that with antennas, "better" is always framed around how well something is able to accomplishing the task at hand.

W8JI's post isn't discussing efficiency, but he is discussing effectiveness. Just because something isn't as effective as we think it is, doesn't mean it is ineffective. It just means that it may not be working as well as it could or should be. Another way of saying ineffective is "doesn't work". Clearly the Imax "works". But it can't work as effectively as a 5/8 GP typically should or could work because it doesn't have the groundplane at the base. Remember this part of W8Ji's post: "The reflection out some distance is how the antenna works and gets gain. Without that groundplane at base height for some distance, the feedline will radiate and the pattern also will not form properly." Nowhere does he say that the antenna won't work. It just won't be the perfect 5/8 wave GP model that everyone expects. And it may not work as well as a simple 1/4 GP.

Here is a link to his page where he shows the effect that groundplane can have on both 1/4 wave and 5/8 wave antennas. This is specifically modeled in a mobile environment, but the principles can be applied:

5/8th wave mobile antenna vs 1/4 wave

 

[jazzsinger]

i think the main point w8ji was making is even the poorest antenna radiating maybe only a few watts from a 100w radio output, can allow rf to propagate sensational distances under the right conditions,

people are very easily impressed by others claiming countries worked when they haven't done it themself, but once you've done these staggering distances on fairly modest priced/homemade reliable gear you begin to realise that just about anything will give good to excellent results sometimes, but only a few sound designs will give good performance in tougher conditions.

those who claim an antenna must be good because it gives a low swr on a few bands outwith its design band and will sometimes hook them a decent contact or two on those bands fail to realise how much better/more reliable an effective antenna designed for the particular band is.

 

[Captain Kilowatt]

There are perhaps some simpler ways to state the point W8JI is making. Most people look at a model of well designed 5/8 wave GP antenna, modeled either in free space of over perfect ground and see a radiation pattern they like (for whatever reason) and see that it has some gain over other designs. One of the designs they often compare it to is a 1/4 wave GP because that is also a common antenna. They then leap forward to the conclusion that the 5/8 wave antenna is always better than the others, which is not the case.



This is the part that people miss. Where do you want the RF to go and does the antenna system that you are going to use accomplish this? Here's a real life example that Happy_Hamer shared with me a couple of years go. He was trying to hit a 2 meter repeater on top of the Sears Tower from his car while in downtown Chicago that had a 5/8 wave antenna on it. He couldn't reliably hold the repeater. He switched it over to a 1/4 antenna and had success every time. Why? because the 1/4 antenna put the RF in the direction that he needed to talk. The point is that with antennas, "better" is always framed around how well something is able to accomplishing the task at hand.

W8JI's post isn't discussing efficiency, but he is discussing effectiveness. Just because something isn't as effective as we think it is, doesn't mean it is ineffective. It just means that it may not be working as well as it could or should be. Another way of saying ineffective is "doesn't work". Clearly the Imax "works". But it can't work as effectively as a 5/8 GP typically should or could work because it doesn't have the groundplane at the base. Remember this part of W8Ji's post: "The reflection out some distance is how the antenna works and gets gain. Without that groundplane at base height for some distance, the feedline will radiate and the pattern also will not form properly." Nowhere does he say that the antenna won't work. It just won't be the perfect 5/8 wave GP model that everyone expects. And it may not work as well as a simple 1/4 GP.

Here is a link to his page where he shows the effect that groundplane can have on both 1/4 wave and 5/8 wave antennas. This is specifically modeled in a mobile environment, but the principles can be applied:

5/8th wave mobile antenna vs 1/4 wave

Good post Tim. People do tend to get all caught up in the take off angle thing and don't realize that sometimes a low TOA is not what is needed or desirable. HH proved that with his repeater problem. I used to use a 7/8 wave collinear on 2m and it worked fantastic for reaching out to far away repeaters or on simplex. It sucked when in close to high repeaters and a simple 1/4 wave worked better. There are times when a high TOA is what is needed to cover a shorter DX contact as well.

Also you were quite right in saying that effectiveness and efficiency are two completely different things. An antenna can be quite effective but not very efficient as is often the case with very low band antennas like VLF antennas or they can be very efficient but not very effective as in very small, well built magnetic loop antennas. A properly built mag-loop maybe 90+% efficient but have a smaller signal than a simple dipole.

 

[Captain Kilowatt]

i think the main point w8ji was making is even the poorest antenna radiating maybe only a few watts from a 100w radio output, can allow rf to propagate sensational distances under the right conditions,

people are very easily impressed by others claiming countries worked when they haven't done it themself, but once you've done these staggering distances on fairly modest priced/homemade reliable gear you begin to realise that just about anything will give good to excellent results sometimes, but only a few sound designs will give good performance in tougher conditions.

those who claim an antenna must be good because it gives a low swr on a few bands outwith its design band and will sometimes hook them a decent contact or two on those bands fail to realize how much better/more reliable an effective antenna designed for the particular band is.

George, you are 100% correct. One common error I see is a newcomer from New England for example, claiming how great his G5RV is because he gets a good SWR on all bands and he just worked a string of Europeans and a few African stations as well as a string from the Caribbean. Sounds impressive to a newbie but not much to someone that has more experience with different antennas. I have done all that on a mobile station with 100 watts.Anyone that has run a G5RV and then switched to a "real" antenna will realise just how ineffective the G5RV can really be especially on 40 and 80m.

 

[HomerBB]

You fellows, Moleculo, Jazzsinger, and Captain Kilowatt are making a point that I seem to have run into with my use of a 5/8 and my Qv4k vs the Astroplane. I actually got better results into the surrounding hills from the AP. To the point that I am determined to redo my homebrews in due time to be sure it wasn't design flaws instead of just a better suited antenna in this local terrain. I suspect it's the latter.

 

[C2]

HH's experience is interesting. Unfortunately, not supported by W8JI's article, and the radiation patterns are insignificantly different.

"The 5/8th wave is never really worse, explaining why people have no problems using them. This is true for any repeater antenna height."

I suspect that someone else could have exactly the opposite expereince...

 

[Captain Kilowatt]

Try working satellites from the mobile using a high gain collinear or even a 5/8 wave versus a simple 1/4 wave. The 1/4 wave has better horizon-horizon coverage due to the pattern. T overhead null is not as deep or as wide.

 

[Moleculo]

HH's experience is interesting. Unfortunately, not supported by W8JI's article, and the radiation patterns are insignificantly different.

"The 5/8th wave is never really worse, explaining why people have no problems using them. This is true for any repeater antenna height."

I suspect that someone else could have exactly the opposite expereince...

Try working satellites from the mobile using a high gain collinear or even a 5/8 wave versus a simple 1/4 wave. The 1/4 wave has better horizon-horizon coverage due to the pattern. T overhead null is not as deep or as wide.

I called HH to make sure I had the facts right. I was a bit off. The antenna he was using at first was a Comet SBB-7 which is a 3/4 wave center loaded antenna on 2m, which is pretty close in size to 5/8 wave. He had it mounted with a lip mount on the back corner of his Jeep. Obviously that location and installation does not provide a very ideal groundplane. If nothing else, the installation and location demonstrate the "unpredictable" results that W8JI was commenting on in the post.

Keep in mind that in the link I gave to W8JI's site, all of the car installation models are used with either a flat roof or curved roof, with the antenna right in the center. It would be interesting to see what it looks like on the rear corner of a Jeep!

 

[bob85]

eddie,
i meant your new top-one model, when you added the mast the the high angle lobe dominated which is what w8ji talks about, mayne i was half asleep and misread it,

cebik's picture show answers what a worst and best case scenario would be both grounded and ungrounded,
neither of them state that there will always be an issue, both of them say you should use a choke,

i doubt that avanti's plot of the astroplane is freespace,
the way i read the patent they did actual measurements and determined that using a mast shorter than about 9ft below the hoop caused takeoff angle to rise significantly,
they also stated that reducing the flare of the radials lowers radiation angle below the horizon,
it is unclear from the patent exactly why the mast causes that effect,
we have a difference of opinion on what the astroplane could be,

there is not more to my ideas about eznec than i let on lol,
i believe if the correct inputs are given then you should get a pretty accurate result within the limitations of each program,
why do i think that? because the experts say so and tens of thousands of people use models to good effect,
cebik gives examples of where mininec or nec could be a better choice in his opinion,

i don't believe people take as much notice of what i post as you imply nor can i imagine why something i posted about a certain type of antenna been difficult to model and get accurate results as told to me by cebik would stop anybody discussing models, if anything it would generate discussion from people who have an interest,
i have posted links to w8ji and cebik that have generated discussion,

several people have posted models of the sigma style antennas we even have shockwaves cst plot from sirio, possibly the only models on the net of that style antenna,
wwrf is certainly the site with the most discussion on the sigma style antennas
the stick and groundplane models are everywhere you look,

henry has it spot on in his summation of what i have said,


you would have to ask the guys that model antennas why they don't share your enthusiasm for posting and discussing eznec models.

 

[MrSuburban]

End-fed Vertical and J-pole

Imax

 

[BOOTY MONSTER]

"Most of the radiation is up in the sky at a high angle. The angle is so high, it is even useless for skywave.

This is a NEGATIVE gain antenna at low angles. A 1/4wl groundplane would seriously outperform the I-MAX 2000 or any other 1/2 or 5/8th wl antenna that does not have a large groundplane.

This pattern is over real earth, where a conventional dipole has about 8 to 8.5 dBi gain. This antenna about -2 dBd gain maximum. It has negative gain over a dipole. The gain over a dipole at most useful angles for DX is about -10 dB....significant negative gain."

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

im no imax or 99 fanboy , but there are tons of folks that use them and have compared them to maco and other 5/8 antennas with 1/4 wl ground elements and the imax excelled both locally and skip wise for them . and theres others that had the exact opposite results with the imax loosing .

seems to be a contradiction between w8ji's post and the real word results of a lot a folks . or im misunderstanding something .

 

[eagle1911]

I've actually been frustrated a few times before because of the wide range of experiences between users of identical antennae. Each installation is unique, pretty much without exception, and there are so many environmental variables that can affect your performance, some of them very subtle but affect things greatly. The propagation conditions vary hugely as well so each day will produce different results, sometimes every hour even. I've had what I consider to be some excellent antenna installations in the past, and there have been times where I would swear I couldn't talk down the street, both transmitting and receiving. For me antennae are one of the most difficult parts of radio to fully understand. That's not to say that you need to have full understanding to get good results, but there are a plethora of variables involved in predicting their behavior.

 

[bob85]

bootie,
w8ji modeled the imax with a worst case mast or feedline length scenario that promotes maximum common mode radiation.
i think you missed his opening statements,

End-fed Vertical and J-pole

when i tested my imax vs the i10k on a short mast the imax was way behind, it was also behind the sirio 827 ( no skip )

i took the imax portable on a 10ft flagpole, performance was reported as been about the same as my 1/4wave whip on the car roof and i could not use the amplifier because of high common mode currents badly screwing up my ssb audio, a choke did not help,

on the other hand i have also had decent results from both the a99 and imax,
on a 1 wavelength pnumatic mast i experienced the old a99 equal my sirio 2012 5/8wave to the majority of my locals.