Re: Amateur Radio Venture Crew
- look hard at Magnetic loops. here is one I did a search on http://www.magneticloopantenna.com/ many more oout there.
--- In ScoutRadio@yahoogroups.com, "KB3CVS (Tom)" <kb3cvs@...> wrote:
> We have a newly formed Venture Crew (Crew 7388 in the Baltimore MD
> area), and we are being asked for many live radio demonstrations and
> merit badge work for area Troops (which is fantastic!)
> Question for the combined wisdom available in this group: What type of
> antenna system would you recommend for HF demonstration capabilities?
> We are often inside, and are unable to get a large antenna setup quickly
> enough. Do other groups do this? If so, what do you use?
> Thanks in advance for any comments!
- --- In ScoutRadio@yahoogroups.com, "KB3CVS (Tom)" <kb3cvs@...> wrote:
> ... we have a new Scout Venture Crew 7388 in the Baltimore MD area ...
> ... What do any of you use here for effective demonstrations?
> We find we need some sort of portable HF antenna which
> is quick and easy to setup, but also effective...
I recently took part in a demonstration for our amateur radio club which may provide you some ideas for the INDOOR part of your problem. The purpose was to demonstration Ham Radio Deluxe, particularly as used with PSK31. The problem is that we meet at a building in which we have never been successful at receiving HF signals, or even the local repeaters other than our own located less than 1/4 mile away.
Three of us were working on the demonstration, and we decided to test out out equipment from our homes a few weeks before the demo. We used 40M for a three way QAO and telephone to co-ordinate and work out the bugs. I was using my FT-817ND QRP rig, MFJ-971 tuner and a dipole using two 40M mobile whips on a PVC pipe supported by a plastic stand left over from a rope light Christmas tree in my living room. As I said, all the equipment was in my living room, with the antenna about two feet below the ceiling. The initial thought was we would set up three stations around a large meeting room in the building. The demonstrator would use my dipole and the other two stations would transmit into dummy loads that would "leak" enough to be heard by the demo station. That would give us as much of an opportunity to hear any "live" stations as possible from inside the building where we hold our meetings. Just to prove that it could be done, after our mutual testing, I answered a CQ from Wisconsin, about 380 miles away, and had a short chat, using only one watt of RF power.
We then reconsidered our demonstration, instead of using the dummy loads, thereby having two deaf transmitter only stations, we would try to use low power into floor mounted mobile whips. I had the two that I had used for the dipole and the presenter had a Hustler mobile with 40M coil on a mag mount that he uses on the metal roof of his "shack". I set out to make two floor mounts for the mobile whips while the presenter fashioned a floor mounted metal plate for his mag mount.
I am sort of the antenna guru for our club, and get teased a lot about using PVC, but my gear works well enough to get used. ( at Field Day 2009 I provided four antennas and masts for our 10A entry ) I had more of the surplus Christmas tree indoor stands that came with the forest of rope light trees I put up in my yard. Each will take a metal tube the size of 1/2 inch electrical conduit. For each stand, I cut about 6 - 8 inches of conduit and attached a "CB type" antenna mirror mount. When I first set up one of the mobile antennas in the mount, it was too tall for my basement ceiling. It was not a problem for a typical office or school building. I could have easily shortened the adjustable portion of the antenna since I intended to use a tuner on the system. However, I had on hand from previous experimentation with these mounts large washers, alternately metal and plastic, cut from milk jugs, that formed a pivot. I reset the mount on the conduit to be horizontal, attached the pivoted mount with the antenna connector, and used the system in that manner.
The day of the demonstration, we could not use the large room, so we set up with roughly 20 feet space between the three mobile whips. I am not sure how the presenter set up his antenna system, but it was sufficient for the demo. I set up the two floor mount stands, each with the 40M whips. I had two 50 foot rolls of coax to connect to the two rigs. I connected to the mount and ran about 6 - 8 feet from the mount, leaving the bulk of the coax coiled on the floor, then ran as much as needed to the rigs. The outer shield of the coax served as a counterpoise or single short radial for the antenna system, with the coiled coax serving as a choke to keep the RF from getting all the way to the transceiver. I was using my FT-817ND set for 0.5 Watts and the other two stations were FT-857s which could only be throttled back to 5 Watts. I used a manual tuner which I set with an MFJ-259 antenna analyzer. The other stations each had auto tuners and I was seriously concerned that their limited tuning range would be a problem. The first attempts to use an auto tuner failed, but before I could dig out the second manual tuner that I had brought along, it was discovered that the failure was not due to excessive missmatch, but rather that the auto tuner needed more than 5 watts to sense the reflected power. My quickie design, without radials or matching circuits could have been used at low power level.
As it turned out, there was so much ambient noise in the building we could hear nothing but ourselves. The building has lots of computer and networking equipment as well as telephone, so even in the parking lot we could not have picked up any live contacts. So we could have just used the dummy loads. Although, I might have needed more than 0.5 Watt.
Given the chance to plan the demonstration again, I probably would have tried to use three QRP tranceivers on 2 meters in a much larger room and brought along an audio recording made from a home station to specifically demonstrate various band conditions.
For those that do not have the generous supply of Christmas tree stands that I do, I would suggest an 8 inch long block of 4 X 4 drilled with a hole for 1/2 metal conduit. Attach the block to a square of plywood or panneling, two feet on a side should work.
It is possible to work HF stations with indoor antennas, providing the building isn't too noisy, but outdoor antennas are going to be a lot more dependable. They are still going to have problems with the ambient noise. I have had a lot of success with monoband quarter wave verticals with four elevated radials. For for a local club's use during Field Day 2000, using PVC pipe and 7/22 #14 stranded antenna wire, I built a set for 20M, 15M, and 10M that worked as well as the G5RV they hung in the trees. That work could be duplicated by Venture Scouts, although today I would have to say that they certainly lack the convenience of the fiber glass poles that I use on their successors.
For wire dipole antennas, the supports that I prefer to use are military surplus aluminum poles intended to support camouflage nets. Each section is four feet long and they have about 3.5 inches at the top which fits inside the next section. I typically use three stacks of 8 for a flat installation, or 10 for the center and 5 at each end for a slightly inverted Vee. One set of guylines is sufficient for 8 or fewer sections. I have also used these poles to support verticals, the tallest at more than 50 feet is a 15M made with aluminum downspout, uses two sets of guylines on the pole with the four radials of the antenna serving as a third set of guylines. There is a PVC section that serves as an interface and insulator between the pole and the downspout.
When possible, I use a large crew to raise and lower the antennas, but with only one exception I have put up by myself with varying amounts of help from extra ropes, pulleys and ground stakes. In all cases, SAFETY is my biggest concern. What I can do by myself requires far more effort, though often less time, when working with others. The only equipment broken was when a helper, contrary to my instructions, walked with a pole that was being lowered thereby allowing slack in a side stabilizing line. The assembly swung out of control and fell causing minor damage. I could not be certain that it would not have fallen if the helper had stayed at his assigned place and kept the slack out of that line. I spend a lot of time thinking about how to build antennas and masts that are simple and strong enough to be put up with a minimum of risk to the equipment and far less hazard to the users.
You, or any of the other members of the group, are welcome to contact me about any of the projects.