6-20m Vertical Radials question

[Moleculo]

Here's something I've been pondering for a while, so I thought I'd post it up here and get some input.

On my roof is a 30' mast with a Cushcraft R6000 vertical mounted on top, multiband dipole just below. Total height to ground from feedpoints is 42'. The Cushcraft is one of their "no ground radials" required antennas. It has a bunch of short radials just above the feedpoint and a matching system.

I've been contemplating whether or not there would be some benefit to laying down a bunch of radials attached to the base of the mast and fan them out on the roof? Even though the Cushcraft is a "no ground radials" required antenna, wouldn't it still make a difference? (My secondary question is whether or not a dipole cares about radials...I have never had a reason to dig deeper on that one.)

Let's say that the radials fanned out on the roof would make a difference; how would I be able to measure the difference or know for certain?

 

[Captain Kilowatt]

Here's something I've been pondering for a while, so I thought I'd post it up here and get some input.

On my roof is a 30' mast with a Cushcraft R6000 vertical mounted on top, multiband dipole just below. Total height to ground from feedpoints is 42'. The Cushcraft is one of their "no ground radials" required antennas. It has a bunch of short radials just above the feedpoint and a matching system.

I've been contemplating whether or not there would be some benefit to laying down a bunch of radials attached to the base of the mast and fan them out on the roof? Even though the Cushcraft is a "no ground radials" required antenna, wouldn't it still make a difference?

All verticle antennas will benefit from radials. The "no groundplane" claim is merely addressing the fact that a suitable impedance match can be obtained without one.Then again a dummy load will do the same thing .

(My secondary question is whether or not a dipole cares about radials...I have never had a reason to dig deeper on that one.)

A dipole will perform better over good conductive ground than it will over lossy ground but if mounted horizontally the addition of radials will not amount to anything unless they extended several wavelengths long.

Let's say that the radials fanned out on the roof would make a difference; how would I be able to measure the difference or know for certain?

You would have better peformance on long haul contacts and would notice more of them however due to the variables of propagation it may be hard to distinguish what is really responsible for the differance.It would be best if you could take field strength readings before and after from several wavelengths away.One thing to take note of is that when adding radials to any verticle that already has a perfect SWR,you will see a point where the SWR will actually start to rise but do not stop adding radials. The reason is that the impedance of a 1/4 wave verticle is ariound 35 ohms over perfect ground and that will translate to an SWR of 1.5:1.The slight rise in SWR is more than offset by better performance. In most cases you can retune the antenna to achieve a 1:1 after adding the radials.

 

[hookedon6]

My secondary question is whether or not a dipole cares about radials...I have never had a reason to dig deeper on that one.)

radials, not really, however, a counterpoise below it will work wonders

 

[Moleculo]

One thing I forgot to mention about the dipole is that it's an inverted V.

OK, back on the topic of the R6000 vertical:
This antenna is not a 1/4 wave vertical. It's linear loaded & trapped antenna. It has 1/4 wave tuning stubs each for 6, 10, and 15 meters and traps for 17 & 20m After I started this thread, I remembered something that is in the manual which conflicts with the typical thinking on radials for verticals:

Cushcraft mentions the following on page 1 of the manual. On the second page, there the following section:

LOCATION​

Although the R6000 will operate in almost any location, it will perform best if it is mounted vertically and located in the clear away from surrounding objects such as buildings, trees, power lines, towers, guy wires, antennas and metallic objects. The R6000 should not be attached to a ground radial system. Failure to heed these points will possibly degrade performance, detune the antenna and increase VSWR. (Emphasis mine)

Now why would that be?

One thing that drives me crazy about this antenna is that the SWR is constantly changing, especially when it rains. I assumed it was because water was getting into the matching device, but then it started changing even when it didn't rain. I thought this could be because of changing ground conductivity. I wouldn't expect that, though with the feedpoint at 42' and the mast mounted on a roof. I also thought that the dipole below it might be causing it, but I don't see why it would. It's not that big of a deal since you can always just touch it up a bit with a tuner, but I was thinking that perhaps I could stabilize it with radials. However, now I see that statement from Cushcraft above and realize I obviously don't understand this antenna.

Here is a link to the manual and product brochure:

http://www.cushcraft.com/comm/support/pdf/R6000.pdf
http://www.cushcraft.com/support/pdf/R6000_master_2_99.pdf​

 

[W5LZ]

Couple of things.
If the SWR is changing for no apparent reason, then the reason is just not 'apparent', it's still there. The trick is to figure out that reason. No idea what to tell you to look for, but if it's like R-7/R-5/etc, there's that matching device to consider. The one on the R-7 used a torroid and a couple of caps in the matching circuit. They tended to get 'squirrelly' at times, water, caps breaking down and so on. Only thing I can think to try is to take it down, clean/repair/etc.
The R6000 is a 1/2 wave if I'm not mistaken, so radials just aren't needed, and will change the input impedance. That's IF they are making electrical contact, not if they are just under the antenna. That'd be just better ground conductivity, sort of. Same for dipoles. It can make a difference, how much, or just exactly how, is dependent on how it's done. Would it be worth the effort? Beats me, try it and see. It will change things. Tuning an antenna depends on what's around it, right? So if you have a more conductive dirt ground (or radials), figure on the resonance changing places a bit, maybe, sort of, or the input impedance changing. How do you quantify the changes those radials produce? Good question! And I don't have an answer. Talk to anyone regularly? See if they notice a difference? Large variables in that 'quantify' thingy huh?
Had a friend who tried that single 'radial' under a dipole. Was able to raise and lower it, and said he did hear a difference. After messing with it for a while, he quit messing with it. Just left it on the ground. Oh well. Try it and tell us what happens.
- 'Doc

 

[wy0mn/7]

Radials under dipoles should optimally be 1/4 wave below them, otherwise its the same sitch as mounting your horizontal antenna 'really' low to the ground. Saw one of those lobed antenna performance graphs once in relation to dipoles & radials. The radial acted as a reflector element shooting wasteful RF 'straight up'.
A google would probably pull that graph up.
Theres a load of info at http://www.bencher.com/pdfs/00361ZZV.pdf but no graph.