Oktober 2012 adalah saat saya, insyaallah, bersemangat lagi untuk 'main' radio. Tentu saja di top band (160m), CW setelah absen dari udara usai kegiatan YB8Y di Kei, Maluku Tenggara, Maret 2012, karena sakit diabet yang cukup serius.
Saya berencana untuk memperbaiki seluruh sistem antena TX jenis vertikal yang, sejak 2008, telah bekerja dengan baik dan memuaskan saya di QTH Cinere, Depok, Jawa Barat. Silakan klik
http://topbanddxdiindonesia.blogspot.com/2008/05/antena-tx-top-loaded-vertical-160m-siap.html)
atau
http://topbanddxdiindonesia.blogspot.com/2008/11/hairpin-coil-untuk-antena-tx-top-load.html
Kesalahan terbesar saya dahulu adalah malas dan tidak membuat identitas pada tiap utas kawat tembaga berdiameter 1mm sampai 2mm untuk ground radials yang - sampai saat menulis ini - sudah berjumlah hampir empat puluh utas. Panjangnya acak dari 15m sampai 40m sehingga satu dan lainnya terjalin (interlaced) dan disolder.
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| Gbr. 1. Sketsa Antena TX 160m dengan Tiang Spiderbeam 18m oleh Rick, DJ0IP |
Untuk memulai pekerjaan itu, semua arsip/dokumentasi catatan saya baca ulang. Agar lebih cepat dan praktis, tampaknya saya harus menyesuaikan instalasi jumlah radials yang lebih sedikit ketimbang jumlah sebelumnya. Namun, instalasi baru hendaknya harus lebih terukur, efektif dan efisien dalam hal panjang dan jumlahnya.
Maka melalui korespondensi e-mail, saya menghubungi beberapa teman lama yang pernah menjadi mentor pembuatan antena TX vertikal 160m di Cinere. Mereka adalah Ken K2KW, Bjorn SM0MDG dan Rick DJ0IP. Untuk menyegarkan ingatan, saya tulis pesan kepada mereka, sebagai berikut:
I am about to
rebuild my T-loaded tx vertival with Spiderbeam 18m fiberglass pole and a hairpin system as impedance
matcher. My question is: how many ground radials to start with, pse?
FYI, my vertical TX antenna is located in the 2nd-storey concrete deck/terrace (about 3,5m above the earth ground). I am planning to have less ground radials -but more accurately measured in the length and amount- than it used to be - which, I think, too many (almost 40 random-length-elevated wires for ground radials)
With existing radials system, my feedpoint impedance (without the hairpin installed) value is at around 12 Ω. Is it good?
Tnx es 73 de Jo, YC0LOW
On Wed, Oct 17, 2012 at 8:58 AM, Kenneth Silverman
wrote:
FYI, my vertical TX antenna is located in the 2nd-storey concrete deck/terrace (about 3,5m above the earth ground). I am planning to have less ground radials -but more accurately measured in the length and amount- than it used to be - which, I think, too many (almost 40 random-length-elevated wires for ground radials)
With existing radials system, my feedpoint impedance (without the hairpin installed) value is at around 12 Ω. Is it good?
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| Gbr. 2. Bracket terbuat dari PVC untuk Sistem Hairpin pada Feedpoint Antena TX 160m |
Tnx es 73 de Jo, YC0LOW
Assuming you are keeping the elevated radials, I'd start with a minimum
of 20. Before you do, measure the
impedance. With the addition of
ground radials, you might see a change
of the feed impedance and it would be nice to track it.
The goal with radials is more about density (distance between radials at the far end) then length, especially when you have short radials. More short radials is better than a few long ones.
12Ω is just where it should be! A full sized vertical should be about 35Ω. When you have a short top loaded vertical 10-25Ω is about right
depending on configuration. If you had a higher impedance (near or above
35Ω) that would indicate loss in the system and coupling to ground.
Sounds like your system is in good shape. 73. Kenny
On Tue, Oct 16, 2012
at 5:39 PM, Björn SM0MDG wrote:
Hi Jo, If you plan to use elevated radials I'd
suggest 2 or 4 radials that are about 3
meters over ground (not critical). I
don't know how big the concrete deck is, but if you plan for radials laying on
the deck I would do as many as possible and start with at least 16. I have a
2-elemet 40 meter vertical array and have 20-24 radials on the ground for each
radiator. That works well, but still not perfect.
Another option is to
check out the FCP, Folded Counterpoise Antenna, that have been discussed on the Topband Reflector.
I have ordered the transformer from Balun Design and plan to try this solution
if time permits, but haven't been able to make it, yet. http://www.w0uce.net/K2AVantennas.html
Hope to hear you in the Stew Perry Warm-up this
weekend!
On Tue, Oct 16, 2012
at 5:29 PM, Rick - DJ0IP / NJ0IP wrote:
Hi Jo, Well for contests, Con DF4SA said he never
use more than 8. I would say 12 to 16.
For home use, I would never use more than 32 because the return on investment is very little above 16 radials.
The more radials you use, the lower the feedpoint impedance will be. It is easy
to match this with a hairpin match.
This is a simple coil
of about 20 to 30 turns of thick copper wire about #12 or #10 wire gauge,
wrapped on a coil diameter of about 3 inches. Shunt this coil across the
feedpoint (the coax) and measure the SWR again. Then pull the coil farther
apart and watch the SWR. Then push it together and watch the SWR. You will see
that in one direction the SWR gets better.You might have to shorten the coil,
or maybe even to add more turns.
I don’t know how to calculate this. I have only done it experimentally as described
above. It is very easy and very quick. If your radials are on the ground and
not elevated, measuring the exact length is almost a waste of time. With 12Ω, you'll need to address the problem.
A 4:1 UNUN would be a
good way of doing this but you will need one with THICK wires because you have
a lot of current at that point. A hairpin match is a much cheaper way of
matching. It will cost you only about
$10 worth of thick copper wire.
73 Rick, DJ0IP
Empat informasi data balasan cepat dari tiga teman di atas saya terima via e-mail.
Rupanya tidak ada konsensus yang dianut oleh mereka untuk menentukan jumlah terbaik kawat radial untuk antena TX vertikal 160m. Isi masing-masing pesan dan/atau tips & tricks-nya dapat Anda baca secara utuh. Saya hanya akan meyimpulkan aspek-aspek kuantitatifnya saja, sebagai berikut:
Ken K2KW: 20 helai kawat tembaga, hairpin system
Bjorn SM0MDG: 16 helai, dengan UN-UN dan isolation
transformer untuk folded counterpoise (FCP)*
Con DF4SA: 8 helai (QSP DJ0IP)
Rick DJ0IP: 12-16 helai, UN-UN atau hairpin system
Rambut sama pirang, namun isi kepalanya berbeda-beda. Yang mana yang saya pilih? Stay tuned.
* Sistem FCP yang ditemukan oleh Guy, K2AV, memungkinkan hampir semua orang dengan lahan yang sempit untuk bisa beroperasi pada top band (160m). Saya pun tertarik untuk mencobanya dan sedang menyiapkan bahan-bahannya serta tulisan tentang sistem FCP dalam naskah terpisah untuk pembaca blog ini. Silakan check-back.
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