YB Land DX Club DX-ing Is Our Life
“…We, engineer are really easy to believe, just show us the data…”
Part-1
When it comes to inexpensive, easy, all around, resonant wire antenna, inverted-Vee configuration become the gold standard for low-bander amateur radio operator. It provides good gain (we will see that later), good coverage (at least medium distance, up to 5000 km), and the-best-bang-for-your-money. Without any doubt, wire dipole antenna on the inverted-Vee configuration can give you good performance under a good installation condition. And of course under good propagation condition, it can deliver good result for intercontinental DX contact. Personally I have seen many successful QSO on 40-m band between Oceania and Africa or Europe which both stations using Inverted-Vee and 100 watts (at least that’s what they said). Several years ago, OM Seno YB2LSR punctured trough a pile-up to 4A2A (Mexico) and VKØEK (Heard Is.) on 40-m band using his old-trusty inverted-Vee antenna and 100 watts radio.
Unfortunately, mainly due to ‘big’ lambda on 40-m band (even more on 80-m band), there are many cases that inverted-Vee antenna are installed under non-ideal condition such as low height above ground (relative to wavelength). Typically most amateur radio operator who categorized to ‘newcomer’ on 40-m band installed inverted-Vee antenna at height of between 6 to 10 m above the ground. It can be understood because of a reason that is such height only need one or two steel pipes to be installed (no need for a tower) and it gives good results for domestic contact. The main problem of such installation is that it only gives you good gain at nearly straight up radiation angle (NVIS radiation, NVIS = Near Vertical Incident Skywave). Because of the mainly NVIS radiation, low height inverted-Vee antenna can gives very good signal strength for domestic/regional contact (up to 1000 km).
Another antenna which is also (relatively) simple and can gives you suprisingly good result but often overlooked is the vertical antenna. It has all good and bad reputation at the same time. Sometimes people said that vertical antenna is weak-to-all-directions and sometimes another people said vertical antenna is surprisingly good so that he can worked dozen and CFM several of new DX entities on low-band from a city lot. Same as the inverted-Vee antenna, mixed performance reports for vertical antenna is due to variety of installation conditions. The most common sub-optimal installation condition for vertical antenna is unproperly made ground system that is basically the half part of the antenna. Such condition gives you excessive ground losses.
Inverted-Vee and Vertical Antenna, What are they
Let’s begin with inverted-Vee antenna. Inverted-Vee antenna basically a form of horizontal wire dipole antenna that are bent to an angle at the feed point. From the picture bellow, everyone can understand the simplicity and why it can be so popular.
With proper size and proper apex angle, it can be matched easily to a 50 Ohm coax transmission line. If installed at 10 m above good ground, it can provide about 5,4 dBi of peak gain. But, that reasonable gain figure is mainly directed nearly straight up. Here are the calculated radiation pattern of 40 m band inverted-Vee antenna at 10 m above good ground.
Even though the main radiation is directed nearly straight up, there are some usable radiation gain (-3 dB down from peak gain) at elevation angle of about 30 degrees from horizon. That is why this antenna is still capable of contacting DX, it’s just require some additional operating skills, patience, and luck.
For some people who got lucky enough to install his inverted-Vee antenna at apex height of (let say) 18 m above ground, the situation is now changed.
If installed at 18 m above good ground, it will provide about 5,7 dBi of peak gain (slight improvement). But, that gain is mainly now directed to a lower angle. Here are the calculated radiation pattern of 40 m band inverted-Vee antenna at 18 m above good ground.
Now the main radiation is directed to elevation angle of about 40 degrees from horizon. And still, there is plenty of usable radiation gain (-3 dB down from peak gain) at elevation angle of about 20 degrees from horizon. The comparison of gain pattern between installation height of 10 m and 18 m above ground is showed at picture bellow. The gain pattern for installation height of 10 m is shown in blue and 18 m is shown in red.
If you look closely, at even lower angle (more favourable for DX contact) there is about 5 dB of gain improvement (quite a lot !) for 18 m installation height. These lowering of radiation angle gives a lot of ‘apparent’ gain improvement even though the peak gain improvement is not that great. Hence, for inverted-Vee antenna, higher installation height can buy you much lower angle of radiation.
It’s enough for us to know what the difference of installation height can made, which is the real struggle for putting up a inverted-Vee antenna. Now we enter the vertical antenna.
Vertical antenna is really just a vertical resonant conductor which is fed against a ground system. Actually it is the easiest way for people who want to work DX but don’t have (or cannot put up) any high enough structure to mount his horizontal wire antenna. By design, it will give you low radiation angle gain easily and if the ground system is properly made, it will give you reasonable gain. The drawbacks are vertical antenna gives you low peak gain figure and even lower gain at high angle (for domestic or regional contact). Picture bellow gives you some idea of how to ideally erect a vertical antenna for low band.
It’s need to know that if you plan to mount only few ground radial (8 radials or fewer) you need to elevate the entire antenna above the ground at least 3 – 4 m. (Please read the story of K2KW effort on putting up 6Y2A contest station and broke some records from Jamaica using vertical at all bands !)
Picture bellow gives you a gain pattern comparison between ideally mounted vertical antenna and 18 m high inverted-Vee. Vertical antenna gain pattern is shown in blue and Inverted-Vee antenna is shown in red.
From the picture above everyone should understand why sometimes people said that vertical antenna is weak-to-all-directions. Actually it’s true for local/NVIS contact which is at certain angle the signal strength can be up to 20 dB or even more weaker than inverted-Vee antenna. The fact is that for all radiation angle above about 13 degrees from horizon, the signal from inverted-Vee is stronger, but for really long haul DX contact that require very low radiation angle, the vertical is win by about 3 – 5 dB (from comparison above). This fact explains why some people reports that vertical antenna gave him surprisingly good result for DX contact. With peak gain of inverted-Vee antenna of 5,7 dBi and vertical antenna is only 0,9 dBi (about 5 dB lower), it is quite amazing actually that at lower radiation angle, vertical antenna is better than 18 m high inverted-Vee.
Like the inverted-Vee antenna, there is some struggle to put up vertical antenna to work properly. The main struggle is to put up properly made ground system. While the inverted-Vee antenna needed some ‘real-estate’ upward to give you low radiation angle, vertical antenna needed some space sideways for ground radials. The main fallacy of people who put up vertical antenna but experienced bad performances for domestic and DX, is that he didn’t put up good ground system / ground radials. Sometimes they only put one or two radials and measured good VSWR at feedpoint but actually he didn’t know what he was measure. For ground mounted vertical antenna, with good number of radials (more than 16) the feedpoint resistance should be around 36 Ohm. If you ever measured feedpoint resistance of 50 Ohm or even more (ground mounted configuration), then there is some ground losses. For elevated mount vertical antenna the feedpoint resistance is even lower at around 30 Ohm. But, under some circumstances, it is possible to angle down the ground radials to get feedpoint resistance of 50 Ohm (like YC2YIZ once did with his tower-top vertical antenna) for elevated mount vertical antenna.
Performance Predictions using 4NEC2+VOACAP
Using VOACAP integrated with 4NEC2, we can predict how well our antenna perform under some propagation condition. Using “ITS-HF Gain Table” tool, we will predict how strong is the signal from inverted-Vee and vertical antenna received at some DX location.
Prediction Condition :
Received Signal Strength, @ 40 m band
at KH6 (Hawaii) 11 UTC
at 9M2 (West Malaysia) 12 UTC
at KL (Alaska) 12 UTC
at JA (Japan) 13 UTC
at VK3 (Australia) 13 UTC
at DU (Philippines) 14 UTC
at VU (India) 15 UTC,
at UA0 (Asiatic Russia) 15 UTC, and
at A4 (Oman) 15 UTC.
Calculated for October 2017, SSN = 26, RX antenna 10 dBi isotropic, TX QTH : OI52EF (Java Is. Indonesia), and TX Power = 100 Watts.
Inverted-Vee Antenna Signal Strength Map
Here are some prediction of received signal strength map from 11 UTC to 15 UTC (shown in order). Please note that more darker color (yellow / orange) indicate stronger signal and more lighter color (green / light blue) indicate weaker signal strength. Please click each picture to open for larger view.
Vertical Antenna Signal Strength Map
Here are some prediction of received signal strength map from 11 UTC to 15 UTC (shown in order). Please click each picture to open for larger view.
Plotting the Data
Using data derived from the maps above, we can compare performace between inverted-Vee and Vertical antenna simply using the graph bellow. Please notice that for easy to read, i’ve normalize the signal strength readout to -150 dBm. The data shows quite amazing performance of vertical antenna. Nearly at all locations (except for Hawaii and West Malaysia) vertical antenna beats inverted-Vee by some margin. The data shows that at KL, JA, VK3, DU, VU, UA0, and A4, vertical antenna have an advantages over inverted-Vee by 1 – 6 dB. This advantages shows that at these locations, DX contact are achieved by low angle radiation. From earlier explaination, we can conclude that these DX location is only contacted using low angle radiation bellow 13 degrees. Only for nearby DX contact (9M2 and DU) where signal strength from both antenna is similar, indicate that the required radiation angle is higher.
Conclusion
Performance prediction comparison between Inverted-Vee and vertical antenna is presented. For typical DX contact, vertical antenna have gain advantages of about 1 – 6 dB above inverted-Vee and for domestic or nearby DX contact, inverted-Vee perform better than the vertical antenna. Depends on the situation, we should choose the optimum antenna, whether it is for domestic contact or for long haul DX contact.
Discussions are welcome.
73,
de Handiko, YBDXC#193
source : link
