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Author Topic: Counterpoise radials  (Read 4434 times)
KC0IGL
Member

Posts: 8




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« on: September 10, 2007, 05:07:43 PM »

I’m a newbie to the HF spectrum of Ham Radio and would greatly appreciate any advice you folks could give me.

Ultimately, and this’ll probably be next spring, I want to mount one of the Hi Sierra antennas to the room of my house.  Currently I’m using a Par end fed in a secret location with pretty good results; however I’m really itching to do better.  I live in a fairly active CCR community and from what I’ve been reading  the Hi Sierra looks like the best deal for my situation.  

Hi Sierra provides 10’ radials but from what I understand of radials, should be quarter length of frequency.  So even though ten feet equates out to about 12M Hi Sierra says that should be ok for most frequencies.    

The dilemma is my roof measures 40 x 25, so given the location of the future antenna I could have about 320 degrees of 20M radials and 70 degrees of 40m (orientated north).  Will any of this work for 80m?  I’m not sure how exact a radial would need to be in length for a given frequency. Will the 10’ radials supplied work fine for 10 through 15 or should I lengthen them for 15 because that is band I would favor.  And wire gauges any preferences?  I’m going to use brown insulated wires cause that blends in with the color of the roof, any problems with insulated?  Maximum power is 100watts supplied buy a nearly new Icom 718 and the roof as a maximum height of 25‘.

Thanks and 73’s

Bill  
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WW5AA
Member

Posts: 2086




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« Reply #1 on: September 11, 2007, 07:07:20 AM »

Bill,

Since this is a tunable screwdriver antenna, the radials do not need to be tuned. The key is as dense and large of a ground plane as possible. Basically run as many radials of the largest diameter wire and as long as possible. More smaller diameter radials are more effective than less larger diameter radials. Insulated or not is not an issue. I always use insulated radials sealed at the end for better oxidation prevention. Have fun!

73, de Lindy
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N4KZ
Member

Posts: 711




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« Reply #2 on: September 11, 2007, 10:45:32 AM »

Bill,

Different types of vertical antenna installations require different types of radials. There are two basic ways to install a vertical -- in the so-called ground mounted configuration where the antenna is mounted vertically at ground level or as a so-called ground plane in which the antenna is mounted in the air, either on a tower, a master or a rooftop.

Radials for a ground-mounted vertical do not need to have any certain dimension. Generally, longer is better. The wire gauge and whether it's bare or insulated is not critical. The radials are attached to the ground shield of the coax and stretched out like spokes on a wheel. However, perfection is not required. Some can twist and turn around obstructions on the ground, for example. The reason they don't need to be a specific length is that the radials are detuned by their proximity to the ground.

On the other hand, when mounted in the air, radials become resonant. That is they must be of a specific length. Generally, one quarter-wave of the desired band. However, length is not terribly critical and variance of a few inches seems unimportant. Three or four radials per band make for a good installation on a ground-plane vertical.

From your post, it sounds like you intend to mount the vertical antenna on your roof. That makes it a ground plane. The short radials supplied by the manufacturer might work OK but my experience has been you will get better performance -- and a lower SWR -- if your radials are tuned to a specific band. I used to go to Radio Shack and buy their flat four-conductor antenna rotor cable. I'd cut each conductor for a different band. I'd prepare four of these and when finished, I had four resonant radials for each of four bands -- but only four wires on the roof. It worked well.

Let me add that as much as I like vertical antennas, I am not certain that the vertical you propose to buy will work any better than your current antenna. Your current antenna is probably more efficient and might be more low profile. But it's hard to say with any certainty -- you just have to experiment.

In closing, I might suggest purchasing some of the fine antenna books published by ARRL, CQ and others. The secret of HF success is about 95 percent determined by one's antenna and there are probably hundreds of antenna designs, theories, practices and other information to absorb and put to use over time. This is particularly true when one has antenna restrictions to overcome.

73 and good luck,
Dave, N4KZ
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WW5AA
Member

Posts: 2086




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« Reply #3 on: September 11, 2007, 12:20:18 PM »

This is why new guys get confused....a screwdriver antenna is tuned via the coil and sometimes in conjunction with a matching unit ...that is why the car (ground plane) under it does not need to be tuned! A screwdriver antenna is not a quarter wave vertical.

73, de Lindy
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KC0IGL
Member

Posts: 8




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« Reply #4 on: September 12, 2007, 04:24:15 PM »

Thanks everyone this was exactly what I was looking for!!
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AA4PB
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Posts: 15046




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« Reply #5 on: September 12, 2007, 06:40:43 PM »

that is why the car (ground plane) under it does not need to be tuned
-----------------------------------------------------
Naw. The car works because it is very close to the ground. The fact that the screwdriver has an adjustable loading coil has nothing to do with it.

Just like Dave said, if the antenna is mounted high in the air then the radials should be resonant (1/4 wavelength for each band). It'll work with non-resonant radials but it'll work better with resonant radials.

Remember that the Hi Sierra is a compromise antenna (as are all mobile antennas) to begin with. This is especially true on 75M and 40M where the whip is very short in terms of wavelength. Your end-fed antenna on the other hand is a full 1/2 wavelength long on the band it is designed for. It'll be more efficient on that band (provided it is installed at a reasonable height) than the Hi Sierra. But then the Hi Sierra is easily tuned for all bands. Radio is full of trade offs and you have to make them according to your circumstances.

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Bob  AA4PB
Garrisonville, VA
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