Date: Sun, 29 Sep 1996 14:00:34 PST From: wb2vuo@juno.com (William K Hibbert) To: qrp-l@Lehigh.EDU Subject: [682] The Inverted-L Antenna Message-ID: <19960929.140719.4759.1.wb2vuo@juno.com> Here's another posting from the BARK PBBS...Keith, WB2VUO ================================================================= Page 1 The Inverted "L" Antenna So, you REALLY want to get on the 160 Meter band, but the neighbors won't let you run your 250' long dipole over their swimming pool? And, you say that the State DOT looks down on your attempt to tie the other end across the State Highway? Well, fear not, as there is a compact solution to your MF antler woes, and it is an Inverted "L". [It can also be cut for 80 or40]. The Inverted "L" is a 1/4-wave Marconi, fed against ground or a counterpoise such as radials. The antenna displays a low feed impedance, which allow you to feed it directly with 50-ohm coax cable. The vertical section should be as high as possible, with the remainder of the antenna run horizontally to a support, such as a tree or a mast. The antenna is fabricated from wire, with the heavier the gauge the better. [The larger the wire diameter, the wider the bandwidth]. One method of installation would be to use a mast as the vertical "wire", with the horizontal section firmly bonded to the top of the mast. The wire is cut to a resonant 1/4-wave length, based on the formula: L(Ft) = 234/F(MHz) If your ground is REALLY good, like your house is on stilts over the ocean, a counterpoise won't be needed, but for our GREAT ground here in Western NY, figure on using at least one counterpoise, if not a few. The length of the counterpoise wires will be cut to a 1/4-wavelength by the formula: L(Ft) = 246/F(MHz) Basically, the counterpoise should be high enough to not get in the way, but could be as high as your installation allows. Mine is run around the base of the house, just below the siding, except around the back door. At that point, I ran the wire under the bottom of doorframe. The wire I used is #18 stranded insulated hook-up wire. CHARTS Well, any antenna discussion is not complete without a chart of some sort, so here are the dimensions for an Inverted "L", listed in tabular form: Frequency Radiator Length Counterpoise Length 1.85 MHz 126.5' 133.0' 1.95 MHz 120.0' 126.2' 3.60 MHz 65.0' 68.3' 3.90 MHz 60.0' 63.1' 7.15 MHz 32.7' 34.4' For bands above 40 Meters, an All-Wire Ground Plane can be constructed, but that's another wire antenna project for the future. Give the Inverted "L" a try, and enjoy your limited-space MF/HF operation. 72/73, Keith, WB2VUO, QRP-L #582 Trustee, KB2YTW/B 10 Mtr Beacon (28.2860 MHz) "In the Depths of the Great Bergen Swamp...FN13ac" ------------------------------ Date: Thu, 3 Oct 1996 10:24:43 PST From: wb2vuo@juno.com (William K Hibbert) To: qrp-l@Lehigh.EDU Subject: [939] wb2vuo: Re: The Inverted-L Antenna Message-ID: <19961003.102453.4719.3.wb2vuo@juno.com> This reply got bounced, so I had to post it to the list. Actually, it might be of "general interest, so read or delete, your choice... --------- Begin forwarded message ---------- From: wb2vuo To: bill@205.134.192.32 Subject: Re: The Inverted-L Antenna Date: Thu, 3 Oct 1996 07:44:30 PST Message-ID: <19961003.082919.4719.0.wb2vuo@juno.com> References: <1.5.4.32.19961003044531.0066a9e0@205.134.192.32> That's correct. The overall length is the figure listed. As for "bending" the top, I have done that, and it worked just fine. In fact, the ends of the 160M inverted vee that I run now are "bent". The last 20' on each end go at roughly a right angle to the main direction so as to stay in the yard. One variation, using the masting for the vertical section, is to make the mast 1/4-wave on a higher band (like 80 or 40 M) nad use a trap in line with the flattop. This allows the mast to be decoupled on the higher band, thus working as a Marconi vertical, and the overall system on the lower band. Doug, W1FB had just such a system in QST and the Handbook some time back. The description of your system gives me an idea for a multi-band set-up, in the same lines as yours. I am going to put it on paper, and post it if the theoretical "study" looks good. (I really need the math coprocessor here so my modeling programs will work...) Hope to catch you on the "Top band" this fall/winter... 72/73, Keith, WB2VUO, QRP-L #582 --------- End forwarded message ---------- ------------------------------ Date: Thu, 3 Oct 1996 16:00:23 PST From: wb2vuo@juno.com (William K Hibbert) To: qrp-l@Lehigh.EDU Subject: [989] Multi-band Inverted-L: A theoretical look Message-ID: <19961003.160029.4719.0.wb2vuo@juno.com> The Inverted-L antenna can be a winner, and will fit in may lots that a dipole won't. One of the possibilities with the Inverted-L is making it a multi-band antenna for the lower end of the HF spectrum, and add 160 Meters to it at the same time. Masts of 30 - 40 feet can be installed at most locations using TV antenna hardware, and will be strong enough to survive almost any adverse weather encountered, short of hurricanes and Alien invasions... So, how can we use this to our advantage? By installing a mast at the end of the house/garage or whatever, and insulating it with PVC pipe sections, the mast can be fed as a vertical, against a suitable ground. One could also attach a wire from the top of the mast, running it horizontally out to a tree or adjacent building, and make the system resonate at a lower frequency. Keep in mind, however, that a base-fed vertical or Inverted-L doesn't have to be 1/4-wave long (high). Let's look at the system like this: 62' horizontal wire ---------------------------------- | | | | | 40' mast | | ======================================================================= The overall length of the antenna is 102 feet. This is roughly 0.18- waves on 160 Meters (1.82 MHz), 0.37-waves on 80 Meters (3.56 MHz) and 0.74-waves on 40 Meters (7.04 MHz). The feedpoint will look reactive on 160 and 80 Meters, and resistive on 40 Meters. The reactance will be capacitive on 160 Meters, requiring a small coil in series to match the feed, inductive on 80 Meters, requiring a capacitor in series for matching and can be fed directly with coax on 40 Meters. This was detailed in the Editors & Engineers "Radio Handbook", in many editions, by Bill Orr, W6SAI, under the subject of a Dual-band Marconi for 80/40 or 160/80 Meters... A matching network could be build into a weathertight box, mounted at the bottom of the antenna, an either switched with a DP3T rotary switch, (requiring the op to walk out & switch it), or wit a pair of DPDT relays. The 40 Meter position would feed the base directly with the coax, the 80 Meter position would switch in a 150 - 250 Pf variable in series with the base and the coax, and the 160 position would switch a 10 - 25 uH coil in series, which would be tapped for the proper match. If you wanted to get fancy, one could have a couple of relays for 160 and 80, and remotely match the antenna at a couple of spots in the band(s). A good ground would be needed, but what I used on my 160 Meter Inverted-L was a pair of 1/4-wave elevated counterpoise wires, wrapped around the house about 3 feet up. It worked better than a round stake and buried wire radial system I used to use. If you didn't want, or couldn't use 160 Meters, the overall length could be cut in half, and run it on 80, 40 and 20 Meters in the same fashion... This is a theoretical view, and if any one can model it, I would be interested in seeing the results... 72/73, Keith, WB2VUO, QRP-L #582 EMPS Q=4 S=1 DX=0 Trustee, KB2YTW/B 10 Mtr Beacon (28.2860 MHz) "In the Depths of the Great Bergen Swamp...FN13ac" ------------------------------ Date: Thu, 3 Oct 1996 21:12:49 -0400 (EDT) From: "L. B. Cebik" To: William K Hibbert Cc: Low Power Amateur Radio Discussion Subject: [1002] Re: Multi-band Inverted-L: A theoretical look Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII On Thu, 3 Oct 1996, William K Hibbert wrote: > The Inverted-L antenna can be a winner, and will fit in may lots that a > dipole won't. One of the possibilities with the Inverted-L is making it > a multi-band antenna for the lower end of the HF spectrum, and add 160 > Meters to it at the same time. > A good ground would be needed, but what I used on my 160 Meter Inverted-L > was a pair of 1/4-wave elevated counterpoise wires, wrapped around the > house about 3 feet up. It worked better than a round stake and buried > wire radial system I used to use. If you didn't want, or couldn't use > 160 Meters, the overall length could be cut in half, and run it on 80, 40 > and 20 Meters in the same fashion... > > This is a theoretical view, and if any one can model it, I would be > interested in seeing the results... > Although there is or has been some discussion trying distinguish a counterpoise from a ground plane, the distinction becomes blurred with elevated ground planes, as have been recommended in some of the recent literature. The effectiveness of buried ground planes is limited by the ground penetration of RF that depends upon one's local soils and, of course, the frequency. In the present case, wrapping the house with the "radials" will like be pretty good as an elevated GP/couterpoise, but virtually impossible to model except by the individual whose GP it is, since the radials interact with lots of stuff in the house, telephone and electrical wiring, for example. Inverted Ls have been around a long time and are well documented. Since the current flow is highest at the feed point, most of the current is in the vertical section producing a mostly vertically polarized signal. However, some cirrent flows in the upper section, and some horizontally polarized radiation occurs. If the antena is used on higher bands, then the ratio of vertical to horizontal current changes, as in multiple quarter wavelength antennas, there are mutltiple current nodes. THis is not bad, since at 20 and up, with a 40 foot tower, the horizontal tradiation angle decreases to one that does quite useful work in the dx angle region. Many of the seemingly random wire antennas that folks start out near the ground, run up over a tree branch and then extend to some other tree branch are really inverted Ls with somewhat unanalyzable ground conditions. Ls also appear to work a little better with poor ground radial systems than regular verticals, perhaps because of the horizontal section. But a good ground plane system is recommended. Distinguish the inverted L from a T, which is really a form of capacity hat vertical: if the T is symmetrical, then its horizontal fields cancel and only the vertical element radiations, even though the T-top is needed to bring the antenna to resonance. Hope these notes are useful. -73- LB, W4RNL ------------------------------