5/8 wave antenna question

[freecell]

they're also notorious for cmc, especially when they're coaxially fed. i should ad a caveat to my last post, the number one determinant as to how much of a counterpoise is required is directly linked to whether or not the EFHW will be operated at resonance and the power levels to be used. the further away the EFHW is from resonance the more counterpoise that is required.

essentially, as the EFHW is moved further and further from resonance the feed point moves away from the current node (voltage loop) of the antenna and more and more of a return is required. something to keep in mind if the EFHW is used across a range of frequencies as opposed to a single operating frequency.

 

[loosecannon]

wow, that was alot to read!
everyone having a boring morning? LOL
just kidding around.

thanks for all the responses.
so, i guess the answer is, no? you cant get a low SWR on a maco V5/8 wave without the radials? (and without an antenna tuner)
loosecannon

 

[Master Chief]

so, i guess the answer is, no? you cant get a low SWR on a maco V5/8 wave without the radials

That's what I said WAY BACK at the beginning of the thread.

 

[smokercraft]

Maco

I read that chief way back then, but you guys like to type and I like to read, so that gives me something to do.Keep on!!!!!

 

[Marconi]

MC, I think you can tune the Maco without radials to a low SWR. I wouldn't do it, because I don't think the antenna would be effective or efficient and that is why just reading the SWR is misleading. Read loosecannon's question again. Since you answered the first time, he has added a new wrinkle at the end:

thanks for all the responses.
so, i guess the answer is, no? you cant get a low SWR on a maco V5/8 wave without the radials? (and without an antenna tuner)
loosecannon

 

[lords]

so there is a reason for that being on a maco on just the SWR? or just for the whole package receive SWR and band?

 

[Master Chief]

The ground radials on the Maco is the other half of the antenna and necessary for it to operate as designed.

Why are we reinventing the wheel?

 

[Marconi]

OK MC raised a good point. Answer this question about an EFHW.

If the impedance at the fed end of a 1/2 wavelength element is so high doesn't that suggest we are at or near a voltage node, and if that is so then we should be absent virtually all current at that point. What good will a ground plane at the feed point do if there is no current available at the feed point?

Isn't that why, in practice, an EFHW works without an obvious ground plane or at the very minimum an extreamly short counterpoise attached near the feed point just to balance the small currents available?

We know that the opposite is absoutely true of a 1/4 wave radiator which is fed at a current node.

 

[freecell]

the further the EFHW antenna is operated from resonance the more pronounced the ground currents.

"doesn't that suggest we are at or near a voltage node, and if that is so then we should be absent virtually all current at that point."

only when the EFHW is operated at resonance. current increases as the antenna is moved away from resonance as the current node or voltage loop is frequency dependent. simple measurements will confirm this to be the case.

 

[Marconi]

Right. I don't disagree, but isn't this why a typical EFHW can get by without an obvious physical ground plane, at least within the bandwidth the matching will allow?

It seems to me, that if there is zero or very little current at the feed point in a EFHW then a counterpoise would be unnecessary, because there is little or no current to return within a very large working area of the bandwidth and that is how they can get by without the radials.

Am I wrong?

 

[freecell]

correct. if you're going to operate over a very finite amount of bandwidth or a single frequency then you may be able to get away without any counterpoise but as i understand it the amount of counterpoise required is directly proportional to the amount of deviation in frequency from resonance, X = 0. ground currents only become apparent when X = something other than 0. you get the chance set up an EFHW at resonance and take a ground current measurement where X = 0 and tell me what you see. it's never 0.

in the short excursion from 27.0 - 29.7 (X = 0 @ 27Mhz. @ 10% BW) ground currents can be measured in the hundreds of milliamps (tenths of an amp) at QRP power levels with only a 10% departure either side from the frequency where X = 0. (24.3Mhz & 29.7Mhz.)

 

[W5LZ]

I must be missing something. Since when is almost 3 Mhz a 'short excursion'?
- 'Doc

 

[loosecannon]

wow,

again, lots to read. glad i could provide a basis for discussion for you guys.LOL

there is a local here who says he runs a maco 5/8 wihtout the radials and says his SWR is 1.5 to 1. (on ch.19 where he sits)

i was sure he was running some sort of "ringo" that is a half wave antenna, and he said is was a 5/8 wave maco.
i didnt feel like arguing with a mule, but i was curious if there was a chance he was right.

yes, master chief did answer the question early on.
thanks all,
loosecannon