Base A99 Antenna Feedline Question

[fourstringburn]

THAT explains what went wrong with the Procomm Patriot 12 antenna I once had. When I first put it in the air, the SWR was great, the needle barely moving. Then after tidying things up and taping the coax to the mast, I went inside to enjoy it only to find the SWR was sky HIGH. No amount of adjusting the tuning rings, adding hose clamps to the tuning rings (as some suggested), or trimming the top of the antenna helped. I eventually hack-sawed it into several pieces and dumped it.

In hindsight I remember now that when I first checked the SWR, the coax was pulled off to one side at approximately a 45 degree angle. And only when I taped it to the mast did things go wrong. The Patriot 12 is notorious for SWR problems and so am guessing it is extremely sensitive to the radiating portion of the coax being near metal.

Holy cow.

That's very possible indeed!

You wouldn't put an antenna right next to a 2 story metal shed would you? Above it would be fine though, in fact great.

That's why I suggested making up stand-off brackets to route this section of coax away from the metal mast or use a 9ft. section of heavy wall PVC or any other non-metallic pipe.

If you look at many Ham dipoles configured with a single support like an inverted V and especially ones fed by twin lead feed line like the G5RV antenna, you will see stand-off brackets to route the twin lead feed line away from a metal mast because with this antenna, a section of twin lead feed line is part of the radiating antenna. Like the A99, the G5RV needs a 1:1 choke balun to separate the feed line section from the radiating section.

Apparently, many people are using the A99 without stand-off brackets and choke baluns after 9ft of feed line and have been able to get their antenna to tune, but they are tuning it with compromised values in the equation. To many, as long as the SWR is low, that's all that matters.

Typical insulated stand off bracket, The further way, the better though.

This is similar to how I made my own stand off brackets when I used a G5RV antenna configured as an inverted V.
I used PVC fittings to keep the twin lead feed line straight and from flopping around in the wind. With my current antenna, I switched to a 45 ft fiberglass push-up pole so I don't have to deal with this any more.

 

[Riverman]

That's very possible indeed!
You wouldn't put an antenna right next to a 2 story metal shed would you? Above it would be fine though, in fact great.

That's why I suggested making up stand-off brackets to route this section of coax away from the metal mast or use a 9ft. section of heavy wall PVC or any other non-metallic pipe.

 

[The DB]

A few things about this antenna.

Their is no such thing as an "end fed" antenna. The best you can do is have a "near end fed". It is impossible to feed an antenna at its end without something accounting for the antenna's "other half". I also don't like how we use the words "other half", namely because it suggests that the other part of the antenna is acting like half of the antenna. In many cases it does very little, especially with some designs, but it must exist for the antenna to function. Even if you take a j-pole, which has a 1/4 wavelength matching stub feeding a 1/2 wavelength end fed antenna, the "other half" in this design is actually an imbalance in the stub, which causes the stub to radiate to some extent.

When it comes to this "other half" many people make many assumptions on how big it has to be. The 1/4 wavelength electrical length is the most common of these. However, for antennas that already include a matching system, these antennas don't need that specific length, and in fact, as little as a few inches will do. The size of this "other half" will affect the tuning of the antenna, but that is only because this "other half" is electrically part of the antenna. In the case of the a99 antenna, their is a built in "other half", namely the tube that is used to mount the antenna to the mast. In and of itself, this is more than enough of an "other half" for this antenna. It is possible to isolate this tube from the mast and choke the feed like immediately under this and have an antenna that works. This setup will work fine, aside from being about the worst place to put a choke on this antenna. (Note: when it comes to the size of the antenna's "other half" and matching sections tenability, not all matching sections can account for all sizes of antennas "other halves" so this may also be a limit for some antennas.)

When it comes to choke placement, putting a choke right at the feed point is, as I said above, about the worst place to put it. Chokes are current devices, and this being a half wavelength antenna, the area near the feed point is a high voltage/low current area of this antenna. A choke placed here will have very little (although still not no) effect on weather common mode currents form and how strong they are. The best place to put such a choke is electrically 1/4 wavelength down from the matching section. On this antenna this will mean using less than 9 feet of coax before the choke. Why less than 9 feet? There are two reasons for this. 1) Their is a foot of coax inside the mounting tube that you really should account for. 2) Unless the coax shielding is exposed to the outside elements the coax will have a velocity factor. This velocity factor will vary from one brand of coax to another, at least to some extent. Taking these two details into account, I would start with a little less than 8 feet of coax. Luckily you don't have to be exact, and positioning the choke off a foot or two will still be very effective.

Optionally, if you make a full set of near 1/4 wavelength radials, you won't need to worry about things like common mode currents forming on the mast or the feed line. 1/4 wavelength radials do an awesome job at dealing with common mode currents, as well as stabilize the antenna system as a whole. For antennas with 1/4 wavelength radials, things like isolating the mast and chokes are more or less redundant. Antennas that include a matching system don't need radials to tune and function, but in some ways said radials will benefit the antenna. If you used the antenna before adding such radials, and then after adding a set of radials you notice a change in performance, you had a common mode currents problem before putting the radials on.


The DB

 

[bob85]

There's plenty of info on this forum and others about end-feds like the a99,
every so often the same kind of technobull keeps getting posted about a simple 1/2wave end-fed antenna,

a99 does not use the coax as a stub or counterpoise or whatever you call it in order to work or have good vswr, a99 is not 1/2 over 1/4wave any more than it has 9+db gain,

a99 does not need any mast or coax to be what it is a 1/2wave end-fed with enough metal below the feed-point for return currents & low vswr even when using a cordless analyser with no mast/coax involved,

not all mast or coax lengths cause cmc issues on the a99 but when they do you can fix it with isolation or sometimes just changing the length of your coax or mast..

 

[fourstringburn]

Like always DB, your posts contain a lot of words to essentially say the same thing.

 

[fourstringburn]

There's plenty of info on this forum and others about end-feds like the a99,

According to The DB, your wrong! There is no "end fed antennas".

a99 does not use the coax as a stub or counterpoise or whatever you call it in order to work or have good vswr,

Many end fed antennas do, Also a good SWR doesn't indicate all is perfect!

W8JI says...End-fed antennas are increasingly popular again, at least partly because of compact iron toroid cores. Small soft-iron cores allow compact, easy-to-build, low-power transformers and networks. The combination of lightweight compact matching systems, combined with the installation convince, visual appeal, and installation simplicity of NOT hanging a heavy coaxial feeder from a long span of thin antenna wire, has rekindled interest in end-fed half-wave antennas.


Unfortunately end-fed antennas have also come back with a little misconception. One commonly repeated myth or "theory" is that half-wave antennas, being resonant, do not require acounterpoise. Lack of a proper counterpoise does not mean the antenna will be worthless and not make contacts, it simply means something else replaces the missing counterpoise area. The feed line, as well as everything connected to and surrounding the feed line, become part of the radiating system. This creates three potential problems:


1.) The feed line, mast, and things around the feed line connect into the receiver. This brings noise into the receiver.


2.) The feed line, mast, and things around the feed line become part of the radiator. This brings voltage (electric fields) and current (magnetic fields) directly into the shack.


3.) The feed line and grounding affects SWR and tuning.


Transmitter power levels, feed line length and routing, and the susceptibility of equipment to RF problems greatly influence things we most likely notice. This is why some people (usually with QRP power levels) swear by end-fed half-waves, while others (usually with higher power) avoid end-fed antennas. The reason for that is simple, end-fed half waves have common mode feed line current problems affecting their performance, and these common mode currents cause inconsistency in user satisfaction. End-fed half wave antennas are best for temporary antennas using low power and batteries, far from power mains and noise sources. They are more prone for problems near noise sources or consumer gear, and can easily exceed FCC RF exposure limits with surprising low power levels.

 

[bob85]

Look at the a99, there's 18" of coax plus the aluminum tube to act as somewhere for return currents, the so239 connector is not the feed-point.

 

[fourstringburn]

Look at the a99, there's 18" of coax plus the aluminum tube to act as somewhere for return currents, the so239 connector is not the feed-point.

In a sense, the feed line attaches to the SO 239 and the matching unit is between the S0 239 and the vertical element. The antenna feed point is still at the SO 239. It's an end fed shunt.

 

[bob85]

The coax shown in the pic is inside the aluminium tube between the matching network and so239 connector,
its not like silver rod type antennas where the matching setup is down at the so239 with no coax between them.

 

[Riverman]

What's great about these forums is that there is a wealth of knowledgeable opinions to choose from. Fourstring's take on the subject makes sense to me in terms I can understand and seems to bear out my experience detailed in Post #14.
That's my story and I'm sticking to it.
Besides, you can never go wrong with a good bass player.

 

[The DB]

Like always DB, your posts contain a lot of words to essentially say the same thing.

If your not interested in how I post, you can always not read them. Just because you don't like how I choose to respond doesn't mean others don't benefit from my posts...


The DB

 

[kopcicle]

I could save everyone the trouble but my opinion is well known.
If you absolutely must use an A-99 or clone then do . Use good coax and some common sense . Expect what you get.
If you want an effective antenna use something else.

I'm lazy and did all this once before. YMMV

http://worldwidedx.com/threads/a99-question.220046/page-2#post-601655

 

[bob85]

fourstring i fixed the a99 schematic for you.

 

[fourstringburn]

fourstring i fixed the a99 schematic for you.

Nice! but you still "feed" the antenna first at the S0-239 therefore it is the "feedpopint", just a matter of semantics.

 

[bob85]

The so239 is not the end of your feed-line nor the feed-point nor the place where return currents enter the inside surface of the coax & kirchhoff does his balancing act like most other end-fed antennas,

on the a99 & imax that place is at the end of the feed-line/coax proper, near the top of the tube where it connects to the matching network some distance from the so239 which is no more than a convenient place to connect your coaxial feed-line extension.