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#21
February 16th 14, 01:55 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
wrote:
On Sunday, February 16, 2014 4:36:32 AM UTC-6, Brian Reay wrote: What band or bands are you trying to work? 80m to 10m on this antenna, inc. 60m I have antennas on the vehicle for VHF/UHF That makes it harder trying to do all bands. It's often easier to pick only the bands one uses the most, and compromise on the others. snipped for brevity Thank you for your considered response, none of which I disagree with. However, I am trying to avoid the need for trees etc. and having wires crossing other peoples' pitches. We tend to have fewer trees on European campsites. The vertical is something I have experimented with prior to buying the RV during club mini-field week sessions and had good results with. I previously I ran out a number of radials but this are too much of a trip hazard on a campsite. I can reduce it to one, about 5% longer than the vertical but I was looking for something 'out of the box'. The antenna is on a tripod. |
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#22
February 16th 14, 06:16 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
On Sunday, February 16, 2014 7:55:35 AM UTC-6, Brian Reay wrote:
The vertical is something I have experimented with prior to buying the RV during club mini-field week sessions and had good results with. I previously I ran out a number of radials but this are too much of a trip hazard on a campsite. I can reduce it to one, about 5% longer than the vertical but I was looking for something 'out of the box'. The antenna is on a tripod. If you have to use the tripod, I would probably try to keep the metal under the tripod as dense as possible, as that is where a lot of ground loss is. A lot of short radials are better than a few long ones for the same amount of wire. Some type of screen or mesh that you could lay down might help a bit. I found on mobile whips, the metal under the antenna is really critical. I once ran a length of angle iron across the back of my truck bed behind the rear cab window. It was well grounded, and even had strap that ran from the bed sides to the base of the antenna to make double sure it was well grounded. It was horrible.  I finally came to the conclusion there just wasn't enough metal under the whip, and just being well grounded by a strap was not enough. I moved the antenna over on to the top of the side tool boxes, which are part of the truck, and gave more metal under the whip, and the antenna came alive in it's normal fashion. It was this experience which leads me to believe that trying some kind of bumper hitch mount is basically a waste of time. It may work, but nothing like it normally does on good metal. I have another truck, and it's mount is on the side of the cab with a big GE ball mount. It's the best of the two. That antenna really cooks on that truck. |
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#23
March 3rd 14, 02:14 AM
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Quote:
Here is an idea to try... an off-center-feed L-dipole. It works this way: calculate the resonant length for the frequency of interest plus 3%; make your vertical arm 2/3 long. Make a second arm 1/3 long and mount it 15 to 30 degrees above horizontal Connect inner coax to the long vertical arm. Connect braid to the shorter arm then adjust its length and angle to lowest SWR. A telescoping antenna is useful at this point. You will find that there is one point where the impedance is exactly 50 ohms. This antenna is directional. There will be gain towards the side arm direction and stronger gain horizontally from the side arm. Signals on the back side will be weak. Take some care which way you set up. Get the arms as high off the ground as possible to get lower angle of radiation and less power adsorbed by the ground and the hot side arm out of reach when transmitting. Dick, KK4OBI |
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#25
March 3rd 14, 04:09 AM
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Quote:
An educated person wouldn't need tuners or multiple dipole antenna's to operate. If you understood antenna's as well as you think you understand antenna's, you would realize that there are antenna's out there that does a little bit of everything - very well thank you - and that antenna tuners do not make your antenna resonant - just makes the transmitter happy. There is a antenna out there that I know of that will work everything from 70 CM to 80 meters - with the exception of 15 and 30 meters - without a antenna tuner - it is called an off-center fed dipole... Please - before you attack my post - look at this web site.... HY Power Antenna's - http://www.hypowerantenna.com/produc...er-fed-antenna Before anyone talks any crap - please let me explain.......... I know the man that designed this antenna... He holds 27 US Patents. K3CC I used this antenna a couple of years ago for Field Days and I made hundreds of contacts - 100 contacts an hour on phone - with no amplifier and with the antenna just 30' off the ground. While others were sitting on their hands doing nothing, I was busy in a pile up on 20 / 40 / 80 meters phone... Yes the Kenwood TS 590 also had a lot to do with this, but the kicker was that there was an idiot there that was free-banding - hopping from one band to the next working digital that was causing a lot of interference to the other participants. I chose to just tune the bugger out. Others with lesser equipment were forced to wait until he was done before they attempted to work their bands while I just kept going. There is a real neat video included with the web site that shows the operator using this antenna as low as 6' off the ground.... A man has got to do what a man has got to do!
__________________
No Kings, no queens, no jacks, no long talking washer women... |
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#26
March 3rd 14, 08:28 AM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
"Channel Jumper" wrote in message
... gareth;815654 Wrote: [b]Then run a "shortish" earth braid from the vehicle's chassis to the antenna.[b] You are confusing earth ground with a counter poise.. No, I'm not. To establish a counter-poise, the counter-poise needs to be directly beneath the antenna. No, it does not. You are providing the second half of a pendulum Just hooking some braid to the antenna won't do anything. Yes it will, taken together with the mass of the vehicle, as I suggested. |
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#27
March 3rd 14, 04:51 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
On Monday, March 3, 2014 2:28:21 AM UTC-6, gareth wrote:
Just hooking some braid to the antenna won't do anything. Yes it will, taken together with the mass of the vehicle, as I suggested. It's possible that connecting to the RV may improve performance due to the antenna becoming a perverted dipole of sorts. But as a means of lowering ground losses under a vertical, your solution will prove to be quite poor. But that it would actually act like a dipole seems fairly remote to me. And there is something that seems to confuse people about short verticals used on vehicles and such. And I think could also be applied to short verticals on tripods. I've fairly much proven to myself that varying the length of the metal on the ground side of the vertical usually does not convert it to a different length dipole. Per say.. If this were the case, I would have to modify and re-tune my mobile whips every time I changed to a different size vehicle, or added other metal to the vehicle. If the antenna acted as a dipole, I would expect to need to re-tune in every change of vehicle or mount. This does not happen. These short verticals are still acting like verticals on the ground, with varying numbers of radials, or metal mass, not perverted dipoles slightly above the ground. My mobile antennas are still resonant at the same frequency no matter what vehicle they are on, big or small, and no change if I add extra radial wires to the vehicle, or even connect directly to the ocean, which I have done in the real world when parked next to the Gulf of Mexico. What does this tell me? That the metal under a mobile whip is acting a lot more like a radial system, or even the ground itself, than the other half of a physical dipole. So what else does that tell me? That connecting a braid from a tripod mounted short vertical to an RV is likely to not pan out too well as far as reducing ground loss under the vertical. In fact, I predict it to be fairly useless. |
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#28
March 3rd 14, 06:52 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
On Monday, March 3, 2014 10:51:14 AM UTC-6, wrote:
That the metal under a mobile whip is acting a lot more like a radial system, or even the ground itself, than the other half of a physical dipole. It depends on where it is connected to the vehicle. If connected near the center, the vehicle acts like two opposing radials and radiates mostly omnidirectional. If connected on the rear bumper, it can act like one radial. On my S10 with a rear bumper-mounted 17m hamstick, my signal was quite directional beaming toward the front of the vehicle. I would sit in the parking lot at Intel and aim my S10 mobile antenna system. I often gained an S-unit by aligning the vehicle with my contact. The fact that the S10 was a quarter wavelength long on 17m probably helped. -- 73, Cecil, w5dxp.com |
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#29
March 3rd 14, 08:29 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
On Monday, March 3, 2014 12:52:37 PM UTC-6, W5DXP wrote:
On Monday, March 3, 2014 10:51:14 AM UTC-6, wrote: That the metal under a mobile whip is acting a lot more like a radial system, or even the ground itself, than the other half of a physical dipole. It depends on where it is connected to the vehicle. If connected near the center, the vehicle acts like two opposing radials and radiates mostly omnidirectional. If connected on the rear bumper, it can act like one radial. On my S10 with a rear bumper-mounted 17m hamstick, my signal was quite directional beaming toward the front of the vehicle. I would sit in the parking lot at Intel and aim my S10 mobile antenna system. I often gained an S-unit by aligning the vehicle with my contact. The fact that the S10 was a quarter wavelength long on 17m probably helped. -- 73, Cecil, w5dxp.com I bet it still acted like a vertical near the ground though, instead of a dipole low to the ground. For instance, I bet the hamstick did not require a drastic re-tuning when mounted near the rear vs mounted in the center. So I still think in your case, it was acting more like a near ground mount vertical with a fat radial to the front, rather than a whip at the rear with the car being the other half of a dipole. And strictly speaking, I suppose it is. Sorta.. Kinda.. But it's not acting like one in operation. It's acting like any other short vertical that is near the ground. I saw a bit of directivity when I had the antenna on the rear trunk lid of a Monte Carlo in much the same way as your S10. It favored the forward direction a tad. I don't notice too much directivity on the trucks, but both have the antenna near the center. Anyway, as it applies to the tripod vertical, I wouldn't expect running a braid to an RV as a very effective way to reduce ground losses below the antenna. And the chances of it pairing up with the RV to produce a usable "dipole" of sorts are not likely to pan out. Ground losses will still be high below the whip, and I bet the RV will more likely resemble a big pile of earth a few feet away, than a viable radiating element. :/ Unfortunately, the big pile of pseudo earth will be in the wrong location to be of much help to the tripod mounted whip. |
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#30
March 4th 14, 04:19 PM
posted to rec.radio.amateur.antenna
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RV/Mobilehome RF gnd systems.
On Tuesday, March 4, 2014 5:22:18 AM UTC-6, Jeff wrote:
Also do not neglect the (considerable) capacitance of the vehicle body ground to the the real ground. I was on the top uncovered story of a three-story parking garage made of concrete and steel and the weather was dry. -- 73, Cecil, w5dxp.com |
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