dipole antenna questions

[loosecannon]

which would give the better performance?

a 40m inverted V with no traps, or a standard 40m dipole with traps.

we are assuming that the traps are of good quality.

also, would a fiberglass mast be better than a metal one for the center support pole when building an inverted V?
(performancewise, not asking about durability or wind handling)

and, should a 1:1 balun be used in all cases?
if not, which ones?

thanks for any info,
loosecannon

 

[W5LZ]

Without traps.
No.
No particular need for a balun at all.
- 'Doc

 

[FL Native]

dont kow about the traps.

anytime you can get your antenna away from any metal objects the better. so yes fibreglass would be better.

if you dont use a 1:1 balun or air choke balun. run your coax out about 4 foot @ 90 degrees away from the feedpoint.

 

[loosecannon]

thanks for the replies guys.

i am getting many suggestions of a G5RV jr.

if go with this antenna, and could only erect a 10' mast on my roof, i would be left with 7 feet of the 450 ohm ladder line sloping down the pitch of my roof.
would this adversely affect the antenna's performance?
by a noticable degree?

also, if i went with the G5RV jr, would the fiberglass be superior to a metal mast?

wondering about interaction between the ladder line and the mast.

thanks,
im learning alot,
loosecannon

 

[Master Chief]

Why do you need traps on a monoband dipole?

 

[loosecannon]

i havent decided yet what kind of antenna i am going to go with.

im pretty sure it will have to be an inverted v of some sort.

i want to be able to use it from 40-10m.

maybe im misunderstanding something, but ive read that a mono band antenna with a tuner will outperform a multiband antenna using traps.

the height limit is what is really "limiting" me.

thanks for the replies,
loosecannon

 

[W5LZ]

loosecannon,
Not all monoband antennas with a tuner will out do a trapped dipole. Some certainly can but there are some "but's" that go with that. One of those "but's" is it tends to work much better if you use ladder line instead of coax. Another "but" is that it depends on the antenna having a harmonic on the desired band of use. You might be ahead to do the trapped dipole. I'm not a big fan of trapped antennas but they do work. I'd much rather use a 'fan' dipole, a dipole cut for each band and all connected to the same feed point, no tuner required. If you don't have lots of room for antenna mounting, that may not be a viable alternative though.
G5RV's. They are very nice 20 meter antennas (what they were designed for), not so nice on other bands. If the ladder line can't be run without adequate clearance for it's whole length, don't bother. It will make a difference. Also, find yourself a good tuner, you'll need it. I just don't feel very good about the 'G5RV JR's. If that's all that will fit the situation, then use it. If you have an alternative though, I think you'd be happier with the alternative.
Height. Another one of those 'more is better' thingys. But if you don't have that available, then use what you got. May not be the 'best' in the world, but it's sure better than nothing, right?
It sounds like you are going to end up with a compromise of some kind. So, take a shot at what you think might work and see what happens. Nothing says you can't change it later...
- 'Doc

 

[loosecannon]

thanks doc,

you and i seem to think a bit alike.

i consider all of my antenna installs to be a series of compromises.
until i get that 500 foot tower and a 6 element beam, they all will be.LOL

this house im moving into will be the most compromising situation yet.

wont be able to put up a vertical at all due to property managers.
just renting till downpayment for REAL house is saved.

so since CB will be strictly mobile for the next year or so, i want to be able to shoot some DX on the ham bands.

the height cant be increased.
no trees to use or hide stuff in. (well, a small cherry tree in the back yard about 8 feet tall.LOL)

i think im going to stay away from traps as they are plainly visible whereas a wire may not be.

would i be better off just going with a 20m inverted v dipole, or the g5rv with the ladder line running the last 7 feet at an angle to the earth?

thanks for all the help guys,
loosecannon

 

[W5LZ]

Which would be better? All things considered, I don't really know. The 'G5RV' might be the better option. With the full sized one you at least have the chance at other bands. Strictly for 20 meters, I'd use the dipole, but you have to remember that I'm predgudiced again't 'G5RV's. (Not because they don't work well on 20, but because they have the undeserved reputation of working well on lower bands.)
You have a much better idea of what you can do at your 'new' house, so that's what you ought to do, not necessarily what I recomend.
- 'Doc

 

[Dave_W6DPS]

Why do you need traps on a monoband dipole?

I'm assuming he actually means loading coils to shorten the antenna. A 40 foot shortened dipole can be a lot easier to site than a full 66 feet. Of course you lose efficiency with the coils.

As for G5RV "Jr." versus dipole, I don't know that there is much advantage if you are only interested in 40 meters. The advantage is that it will probably work 40 to 10 meters, maybe 6. With a 40 meter inverted-V and coax you basically are limited to 40 and 15.

Using a fiberglass pole should eliminate the need to isolate the ladderline from the mast. Any metal close to a ladderline can detune it, and you would need standoffs with a metal pole--but that is the way they have been used for a long time.

I prefer a 1:1 balun, to head off feedline radiation and give a more consistent pattern. Of course, a coax choke will usually work just as well, and you don't always need either. If you don't use either with coax you basically have two section to your counterpoise--the feedline outer conductor and the half of the dipole connected to it. You cn get some pretty weird lobes on such an antenna. Of course, if one or more of those lobes goes where you want to talk it is not a problem. Unfortunately there is often a strong lobe shooting mostly to the ground.

In the real world there are a lot of things that affect your radiation pattern, so a balun or choke may not make that much difference. If you have RFI in your house, or a generally poor performance, you might think about a balun or choke to see if that is the issue.

You can also google an antenna called a half-sloper, which is basically an inverted-V with only one side. the transmission line outer conductor and/or the tower are used as the counterpoise. You don't want a balun on this one,a nd the choke would be at the base of the tower if you have RF in the shack.

Dave_W6DPS

 

[loosecannon]

thank you dave,

i sure am learning alot.
all the wire antennas ive put up in the past, (about 5 LOL) have been for receiving only, so i never got deep into the theory.

its looking to me, like the G5RV JR with a fiberglass mast is the way to go for me.
even with compromised performance, i would still like to be able to work 40-10m.

i have read that a balun is not recomended at the transition from the ladder line to the coax because it sees alot of reactance which baluns dont do well with.
here is a quote from one article:

"The antenna can be fed by any convenient type of feeder provided always that a suitable type of antenna tuner is used. In the original article describing the G5RV antenna, published in the , then, RSGB bulletin November 1966, it was suggested that if coaxial cable feeder was used, a balun might be employed to provide the necessary unbalanced-to-balanced transformation at the base of the matching section. This was because the antenna and its matching section constitute a balanced system, whereas a coaxial cable is an unbalanced type of feeder. However, later experiments and a better understanding of the theory of operation of the balun indicated that such a device was unsuitable because of the highly reactive load it would "see" at the base of the matching or "make-up" section on most hf bands.

It is now known that if a balun is connected to a reactive load presenting a vswr of more than about 2:1, its internal losses increase, resulting in heating of the windings and saturation of its core (if used). In extreme cases, with relatively high power operation, the heat generated due to the power dissipated in the device can cause it to burn out. However, the main reason for not employing a balun in the case of the G5RV antenna is that, unlike un antenna tuner which employs a tuned circuit, the balun cannot compensate for the reactive load condition presented to it by the antenna on most of the hf bands, whereas a suitable type of antenna tuner can do this most effectively and efficiently.

Recent experiments by the author to determine the importance or otherwise of "unbalance" effects caused by the direct connection of a coaxial feeder to the base of the matching section had a rather surprising result. They proved that, in fact, the hf currents measured at the junction of the inner conductor or the coaxial cable with one side of the (balanced) matching section and at the junction of the outer coaxial conductor (the shield) with the other side of this section are virtually identical on all bands up to 28mhz, where a slight but inconsequential difference in these currents has been observed. There is, therefore, no need to provide an unbalanced-to-balanced device at this junction when using coaxial feeder."

of course, an unbalanced to unbalanced tuner is necessary, and the coax should be less than 70 feet long according to the article.


thanks for all the help guys,
loosecannon