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Author Topic: Rocket 88 antenna  (Read 8187 times)
K2TL
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Posts: 40




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« Reply #15 on: September 08, 2009, 02:52:31 AM »

First of all, it is not "my" Rocket88.  I don't own one.  I built an 88 foot doublet and feed it with 300 ohm transmitting ladderline.  It is commonly referred to as the Cebik Dipole.
Next, yes coax is very lossey and will radiate if terminated into a mismatch.  The loss is dependent on frequency, length, and the severity of the mismatch.  In fact, the so called Carolina Windom actually depends on the coax radiating as an integral part of this particular antenna.  It is quite easy to lose 3 db on the G5RV's section of coax.  I think that means half your power?
Can a G5RV be built to reduce the loss on the coax section?  I suppose so, but there are alternatives that result in a better antenna.  That does not mean an 88 foot doublet, necessarily.  But the Cebik Dipole is a better choice than a G5RV, as are many other antennas.
Again, all hail the G5RV..if that makes you sleep better.  Not me.
K2TL
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K2TL
Member

Posts: 40




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« Reply #16 on: September 08, 2009, 05:12:08 AM »

In case anyone is interested, here is a site ( one of many ) that discusses the loss on the coax section of a G5RV:

http://vk1od.net/antenna/G5RV/optimising.htm

scroll down a bit to see the chart. The author states "The coaxial transmission line is usually the single greatest contribution to poor efficiency." He makes some suggestions to improving it. If you MUST use an antenna like the G5RV, consider the ZS6BKW version.  Here is one of many sites :

 http://www.nonstopsystems.com/radio/ZS6BKW.pdf


 but do your own Google.
K2TL
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N3OX
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Posts: 8847


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« Reply #17 on: September 08, 2009, 06:20:39 AM »

"First of all, it is not "my" Rocket88. I don't own one. I built an 88 foot doublet and feed it with 300 ohm transmitting ladderline. It is commonly referred to as the Cebik Dipole. "

OK, noted.

"Next, yes coax is very lossey and will radiate if terminated into a mismatch"

*THAT* is an over-generalization and a myth rolled into one.

First of all, it's not termination into a mismatch that causes coax radiation.  That's bad information.  You can put an SO-239 connector in the side of a well shielded box with nothing inside and hook up the well shielded coax and apply power (through a beefy tuner).  This is essentially an open circuit, a terrible mismatch.  But it won't radiate, just like putting a dummy load in a shielded box.  If you look at the "open circuit" for a sensitive spectrum analyzer it's built just like this.  You want the shielded-ness to keep the outside of the coax from coupling to the inside.  

That's what causes feedline radiation.  The outside of the coax is coupled to the inside or the antenna couples EM energy directly to the shield.  With no such  coupling, which happens either by induction or by direct conduction at the junction of the coax and something else, the outside does not know what the inside is doing.

"coax is very lossey"

Coax is as lossy as coax is.  You are RIGHT that termination into a mismatch will typically cause extra coax loss (and the examples where there's less loss aren't really worth mentioning here except to say they exist).  But the amount of loss for a given termination is CALCULABLE and is not always "very lossy".  

"Very lossy" is not a statement that can help anyone pick an antenna.  It's a statement that will irrationally scare people away from one type of antenna only to trade for another that has different sorts of problems.  It seems like good advice in the context that feeding an "all-band" random antenna with coax is invariably a disaster.  But there are some antennas, a good G5RV included, that are *designed* to present reasonable mismatch to coax to mitigate those losses.

Every feedline has losses.  There are common situations in which ladderline is less lossy than coax, but it is not *every* situation nor is it always an *important* amount of loss.

"In case anyone is interested, here is a site ( one of many ) that discusses the loss on the coax section of a G5RV:

http://vk1od.net/antenna/G5RV/optimising.htm

scroll down a bit to see the chart. The author states "The coaxial transmission line is usually the single greatest contribution to poor efficiency.""

Yes, and as I'm sure VK1OD would tell you, that efficiency *can* be CALCULATED and rationally discussed...

Look... I'm not trying to be a big G5RV cheerleader.  There are a lot of poor ones out there.  But what I don't want to see is some guy who has poor performance on 75m taking down his G5RV and putting up a shorter doublet or worse spending money on a new "brand name" antenna  because he thinks "coax is very lossy" or "the mismatch causes the feedline to radiate" when the former is somewhat misleading and can be calculated and the latter is just not true.

And the reason why I feel strongly about this is not because I'm overly fond of a G5RV.  I would actually be putting up a ladder line fed doublet in a heartbeat, with ladder line to ground level transitioning to this:

http://www.n3ox.net/projects/servo

The reason why I feel strongly about this is more that when a ham takes down a bad antenna and replaces it with another type *for the wrong reasons,* then they "learn" all the wrong reasons really well.  There's no teacher like experience.  But if the explanations behind the experience were incorrect or incomplete, they become cemented in place by the experience.

Yes, coax loss is likely the reason why many G5RV antennas are no good.  But switching from RG-58 to RG-8/X and following VK1OD's trimming and tweaking procedure could be a good learning experience and result in an antenna that's just as good on the bands a G5RV works on as a shiny new Rocket 88 would be.

You and I both know that the most important thing in a multi-band wire antenna is this:

1) Put up some wire high and in the clear
2) Couple power to it efficiently

But there are many ways to do that, all with various tradeoffs.  And it seems with the 88 foot doublet, one of those tradeoffs is that a lot of people end up cooking their tuners on 80m when they run high power while dropping a couple dB or worse in the overall system.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K2TL
Member

Posts: 40




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« Reply #18 on: September 08, 2009, 06:57:31 AM »

The G5RV has been marketed as a multiband antenna and has been a money maker to those who sell them.  I think that is why it perpetuates, more than any other reason.  Newcomers, or even non newcomers who don't want to know any better, use them. They work, get you on the air, and will give the user the impression that it is a good antenna. I would not recommend a G5RV to a newcomer.  I believe there are better antennas for very little effort and expense.  I still can't imagine why anyone would purchase a dipole, unless you are so physically handicapped that you can't build something so utterly simple.  Maybe I should get into the business myself.  I'll call it the "K2TL Wonder Wire".
Smiley
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W8BNL
Member

Posts: 21




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« Reply #19 on: December 10, 2009, 01:09:52 PM »

Wow - - two pages of drivel and bickering only to find that there is no one who could answer N8DV's Original question - "Is there anyone running the Rocket 88 antenna from QSradio.com?" So apparently my response to the original poster of this topic is "No, there isn't anyone running a Rocket 88 antenna from QSradio.com." You gotta admit though - it was fun reading through all the banter.

73 de W8BNL - Tom
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KA2CYN
Member

Posts: 1




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« Reply #20 on: July 09, 2010, 06:31:04 PM »

Take antenna makers' claims about like you take nutritional supplement claims.

I would agree except that this claim was published in a QST ad.  Unless their policy has changed they do not allow ad claims that can't be verified.

Dave KA2CYN
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